Fix Redefined Callback (#566)

## Description

RuntimeLibExitBootServicesEventCallback is defined and exported in two
locations, RuntimeLib and PolicyLib. This causes a linker error if both
libraries are linked to a module.

This patch both changes the name of the callback in PolicyLib (as this
was not an accurate name) and marks it STATIC, as this function should
not be called at all except for the function pointer that is registered
for the event firing.

Closes #565.

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- [ ] Impacts functionality?
- **Functionality** - Does the change ultimately impact how firmware
functions?
- Examples: Add a new library, publish a new PPI, update an algorithm,
...
- [ ] Impacts security?
- **Security** - Does the change have a direct security impact on an
application,
    flow, or firmware?
  - Examples: Crypto algorithm change, buffer overflow fix, parameter
    validation improvement, ...
- [ ] Breaking change?
- **Breaking change** - Will anyone consuming this change experience a
break
    in build or boot behavior?
- Examples: Add a new library class, move a module to a different repo,
call
    a function in a new library class in a pre-existing module, ...
- [ ] Includes tests?
  - **Tests** - Does the change include any explicit test code?
  - Examples: Unit tests, integration tests, robot tests, ...
- [ ] Includes documentation?
- **Documentation** - Does the change contain explicit documentation
additions
    outside direct code modifications (and comments)?
- Examples: Update readme file, add feature readme file, link to
documentation
    on an a separate Web page, ...

## How This Was Tested

Unit tests ran.

## Integration Instructions

N/A.

PolicyServicePkg/Library/DxePolicyLib/DxePolicy.c

@@ -68,9 +68,10 @@ GetPolicyInterface (
   @param[in]  Context The Event Context.
 
 **/
+STATIC
 VOID
 EFIAPI
-RuntimeLibExitBootServicesEvent (
+PolicyExitBootServicesEventCallback (
   IN EFI_EVENT  Event,
   IN VOID       *Context
   )
@@ -101,7 +102,7 @@ PolicyLibConstructor (
   Status = gBS->CreateEvent (
                   EVT_SIGNAL_EXIT_BOOT_SERVICES,
                   TPL_NOTIFY,
-                  RuntimeLibExitBootServicesEvent,
+                  PolicyExitBootServicesEventCallback,
                   NULL,
                   &mPolicyExitBootServicesEvent
                   );