Include <sys/stat.h> in src/util.c

On FreeBSD 11-CURRENT this fixes the compilation failure:

src/util.c: In function 'file_write':
src/util.c:256: error: 'S_IRUSR' undeclared (first use in this function)
src/util.c:256: error: (Each undeclared identifier is reported only once
src/util.c:256: error: for each function it appears in.)
src/util.c:256: error: 'S_IWUSR' undeclared (first use in this function)
*** Error code 1
1 file changed