Revert back again fixing a typo! (thanks @springmeyer 2f55f0962e (commitcomment-20461372))

"It doesn't work with GCC reverting back "disable [-Wunused-result] warning GCC : https://gcc.gnu.org/bugzilla/show_bug.cgi?id=25509""

This reverts commit 2f55f0962e.

include/mapnik/util/file_io.hpp

@@ -83,7 +83,11 @@ public:
         if (!size_) return nullptr;
         std::fseek(file_.get(), 0, SEEK_SET);
         data_type buffer(new char[size_]);
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wunknown-pragmas"
+#pragma GCC diagnostic ignored "-Wunused-result"
         std::fread(buffer.get(), size_, 1, file_.get());
+#pragma GCC diagnostic pop
         return buffer;
     }
 private: