Fix file descriptor leak (reported by Coverity)

While we really don't care about it and the descriptor will be
closed anyway when the process gets terminated, it's still better
to fix this.

main.c

@@ -60,10 +60,14 @@ static void *mmap_framebuffer(size_t *fbsize)
         if ((fd = open("/dev/graphics/fb0", O_RDWR)) == -1)
             return NULL;
 
-    if (ioctl(fd, FBIOGET_FSCREENINFO, &finfo))
+    if (ioctl(fd, FBIOGET_FSCREENINFO, &finfo)) {
+        close(fd);
         return NULL;
+    }
 
     p = mmap(0, finfo.smem_len, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+    close(fd);
+
     if (p == (void *)-1)
         return NULL;