Fix getmntent check with -Werror.

[gonsolo]

Fix a warning. The test is just compile tested and the return value is not used anyway.
Also, in my private build I compile with "-Werror" and without this patch this fails.

It also fails to build:
../libs/pbd/mountpoint.cc:114:10: fatal error: sys/ucred.h: Datei oder Verzeichnis nicht gefunden 114 | #include <sys/ucred.h> | ^~~~~~~~~~~~~ compilation terminated.

[gonsolo]

The test program cannot be run. It always fails with a segmentation fault. A better one would be:

FILE *f = setmntent("/", "r");
if (f) { endmntent(f); }

[x42]

The test is not executed, but it needs to compile without warning if OP wants to use -Werror.

[kiilerix]

The current check script will however compile even if mntent.h doesn't contain getmntent. The default declaration of getmntent is fine.

The check could thus be simplified to
msg="Checking for mntent.h", fragment = "#include <mntent.h>",
which I assume also works fine for the use case of -Werror.

A more (pointless) thorough check that checks for valid content of mntent.h could perhaps also do
struct mntent *tmp = getmntent(0);