Revert "Remove pragmas to disable -Wstringop-truncation"

I was compiling with Clang, and not GCC.  Put the warning back in!

This reverts commit 659eeae.

src/bin/tools/mimegen.c

@@ -297,8 +297,11 @@ int main(int argc, char *argv[])
     for (i = 0; i < hash_get_count(ext_mime); i++) {
         uint64_t ext_lower = 0;
 
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wstringop-truncation"
         /* See lwan_determine_mime_type_for_file_name() in lwan-tables.c */
         strncpy((char *)&ext_lower, exts[i], 8);
+#pragma GCC diagnostic pop
 
         ext_lower &= ~0x2020202020202020ull;
         ext_lower = htobe64(ext_lower);

src/lib/lwan-tables.c

@@ -142,11 +142,14 @@ const char *lwan_determine_mime_type_for_file_name(const char *file_name)
     if (LIKELY(last_dot && *last_dot)) {
         uint64_t key = 0;
 
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wstringop-truncation"
         /* Data is stored with NULs on strings up to 7 chars, and no NULs
          * for 8-char strings, because that's implicit.  So truncation is
          * intentional here: comparison in compare_mime_entry() always loads
          * 8 bytes per extension. */
         strncpy((char *)&key, last_dot + 1, 8);
+#pragma GCC diagnostic pop
 
         return bsearch_mime_type(htobe64(key & ~0x2020202020202020ull));
     }