Update build.md

Added pkgconfig to FreeBSD dependencies. Without pkgconfg cmake will result in error
```
CMake Error at /usr/local/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:230 (message):
  Could NOT find PkgConfig (missing: PKG_CONFIG_EXECUTABLE)
Call Stack (most recent call first):
  /usr/local/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:600 (_FPHSA_FAILURE_MESSAGE)
  /usr/local/share/cmake/Modules/FindPkgConfig.cmake:99 (find_package_handle_standard_args)
  CMakeLists.txt:167 (find_package)
```

doc/build.md

@@ -55,7 +55,7 @@ sudo dnf install alsa-lib-devel avahi-devel gcc-c++ libatomic libvorbis-devel op
 ### For FreeBSD
 
 ```sh
-sudo pkg install alsa-lib pulseaudio cmake gmake gcc bash avahi libogg libvorbis opus flac libsoxr
+sudo pkg install alsa-lib pulseaudio cmake gmake gcc bash avahi libogg libvorbis opus flac libsoxr pkgconfig
 ```
 
 ### For macOS