utils: fix public API with consumers using _POSIX_C_SOURCE on FreeBSD

cb6dbd165a made public API incompatible with consumers using
_POSIX_SOURCE, _POSIX_C_SOURCE or _XOPEN_SOURCE: <sys/mount.h>
isn't standalone but depends on BSD-specific types in <sys/types.h>
which are hidden when POSIX compliance is requested.

xdg-desktop-portal-wlr passes -D_POSIX_C_SOURCE=200809L via meson.build
which works for its own code and all other dependencies.

In file included from ../src/core/main.c:7:
In file included from /usr/local/include/pipewire-0.3/pipewire/pipewire.h:35:
In file included from /usr/local/include/pipewire-0.3/pipewire/client.h:35:
In file included from /usr/local/include/pipewire-0.3/pipewire/proxy.h:108:
In file included from /usr/local/include/pipewire-0.3/pipewire/protocol.h:48:
In file included from /usr/local/include/pipewire-0.3/pipewire/utils.h:34:
In file included from /usr/include/sys/mount.h:38:
/usr/include/sys/ucred.h:99:2: error: unknown type name 'u_int'
        u_int   cr_version;             /* structure layout version */
        ^
In file included from ../src/core/main.c:7:
In file included from /usr/local/include/pipewire-0.3/pipewire/pipewire.h:35:
In file included from /usr/local/include/pipewire-0.3/pipewire/client.h:35:
In file included from /usr/local/include/pipewire-0.3/pipewire/proxy.h:108:
In file included from /usr/local/include/pipewire-0.3/pipewire/protocol.h:48:
In file included from /usr/local/include/pipewire-0.3/pipewire/utils.h:34:

Reported by: Jan Beich <jbeich@FreeBSD.org>

src/pipewire/utils.h

@@ -31,7 +31,9 @@ extern "C" {
 
 #include <string.h>
 #include <sys/un.h>
-#include <sys/mount.h>
+#ifndef _POSIX_C_SOURCE
+# include <sys/mount.h>
+#endif
 
 #include <spa/utils/defs.h>
 #include <spa/pod/pod.h>