libsepol: fix unitialized variable 'nread' on mac build

Fix this:
genusers.c:63:14: warning: variable 'nread' is uninitialized when used here [-Wuninitialized]
                if (buffer[nread - 1] == '\n')
                           ^~~~~
genusers.c:40:15: note: initialize the variable 'nread' to silence this warning
        ssize_t nread;
                     ^
                      = 0

Signed-off-by: William Roberts <william.c.roberts@intel.com>

libsepol/src/genusers.c

@@ -53,6 +53,7 @@ static int load_users(struct policydb *policydb, const char *path)
 	}
 
 	while(fgets(buffer, 255, fp) != NULL) {
+		nread = strlen(buffer);
 #else
 	size_t len = 0;
 	__fsetlocking(fp, FSETLOCKING_BYCALLER);