sys/mnttab.h: include sys/stat.h for stat64

Musl libc defined `stat64` as a macro, which causes the build to fail
upon compiling os/linux/getmntany.c due to conflicts between the forward
declaration and the implementation.

This commit fixes that by including <sys/stat.h> in "sys/mnttab.h"
directly.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Hiếu Lê <leorize+oss@disroot.org>
Closes #10195

lib/libspl/include/os/linux/sys/mnttab.h

@@ -32,6 +32,7 @@
 
 #include <stdio.h>
 #include <mntent.h>
+#include <sys/stat.h>
 #include <sys/types.h>
 
 #ifdef MNTTAB
@@ -67,7 +68,6 @@ struct extmnttab {
 	uint_t mnt_minor;
 };
 
-struct stat64;
 struct statfs;
 
 extern int getmntany(FILE *fp, struct mnttab *mp, struct mnttab *mpref);