Fix compile-time warnings on IGNORE_MTAB

Fix compile-time warnings on IGNORE_MTAB

Silence below warnings which appear if IGNORE_MTAB is defined.

[59/64] Compiling C object 'util/fusermount3@exe/fusermount.c.o'.
../util/fusermount.c:493:12: warning: function declaration isn't a prototype [-Wstrict-prototypes]
 static int count_fuse_fs()
            ^~~~~~~~~~~~~
../util/fusermount.c: In function 'unmount_fuse':
../util/fusermount.c:508:46: warning: unused parameter 'quiet' [-Wunused-parameter]
 static int unmount_fuse(const char *mnt, int quiet, int lazy)
                                              ^~~~~