fix: missing -DPROTOBUF_USE_DLLS in pkg-config (#12700)

When the protobuf libraries have been compiled as shared libraries the users of the library need to add `-DPROTOBUF_USE_DLLS` to their build line. Otherwise some symbols are missing.

Fixes #12699

FWIW, I am not sure this is an ideal fix.  It may be better to fix the headers such that no macros change the ABI.

Closes #12700

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/12700 from coryan:fix-define-protobuf-use-dlls-in-pkg-config-file 13c792eebd3d070fa25eff68cfca6ae1493e9503
PiperOrigin-RevId: 530116678

cmake/protobuf.pc.cmake

diff --git a/cmake/protobuf.pc.cmake b/cmake/protobuf.pc.cmake
index 21bbf47..81d87c1 100644
--- a/cmake/protobuf.pc.cmake
+++ b/cmake/protobuf.pc.cmake
@@ -8,5 +8,5 @@
 Version: @protobuf_VERSION@
 Requires: @_protobuf_PC_REQUIRES@
 Libs: -L${libdir} -lprotobuf @CMAKE_THREAD_LIBS_INIT@
-Cflags: -I${includedir}
+Cflags: -I${includedir} @_protobuf_PC_CFLAGS@
 Conflicts: protobuf-lite