backend: fix includes

Fix a regression in commit d1dac54b72 where sys/stat.h is no longer
included in backend_firefox.c and backend_chromium.c.

It has to be explicitly included for the UTIME_xxx macros.

src/backend_chromium.c

@@ -37,6 +37,7 @@
 
 #include <fcntl.h>
 #include <iconv.h>
+#include <sys/stat.h>
 #include <unistd.h>
 
 #ifdef BOOKMARKFS_BACKEND_CHROMIUM_WRITE

src/backend_firefox.c

@@ -35,6 +35,7 @@
 #include <string.h>
 
 #include <fcntl.h>
+#include <sys/stat.h>
 #include <unistd.h>
 
 #ifdef BOOKMARKFS_BACKEND_FIREFOX_WRITE