firmware-utils/dgfirmare: fix possible resource leak

Add missing calls to `fclose` in functions `write_img`, `write_rootfs`
and `write_kernel`.
The not-closed files could lead to resource leaks.

Signed-off-by: Andrea Dalla Costa <andrea@dallacosta.me>

tools/firmware-utils/src/dgfirmware.c

@@ -86,6 +86,8 @@ void write_img(unsigned char* img, const char *fname)
     fclose(fp);
     exit(-1);
   }
+
+  fclose(fp);
 }
 
 
@@ -104,6 +106,8 @@ void write_rootfs(unsigned char* img, const char *fname)
     fclose(fp);
     exit(-1);
   }
+
+  fclose(fp);
 }
 
 
@@ -122,6 +126,8 @@ void write_kernel(unsigned char* img, const char *fname)
     fclose(fp);
     exit(-1);
   }
+
+  fclose(fp);
 }