Add gnulib-fix-gcc7-fallthrough.diff

As long as the code is not upstream, add it as explicit patch for the
case of gnulib refresh.

grub-core/gnulib-fix-gcc7-fallthrough.diff

@@ -0,0 +1,14 @@
+diff --git grub-core/gnulib/regexec.c grub-core/gnulib/regexec.c
+index f632cd4..a7776f0 100644
+--- grub-core/gnulib/regexec.c
++++ grub-core/gnulib/regexec.c
+@@ -4099,6 +4099,9 @@ check_node_accept (const re_match_context_t *mctx, const re_token_t *node,
+     case OP_UTF8_PERIOD:
+       if (ch >= ASCII_CHARS)
+         return false;
++#if defined __GNUC__ && __GNUC__ >= 7
++      __attribute__ ((fallthrough));
++#endif
+       /* FALLTHROUGH */
+ #endif
+     case OP_PERIOD: