Fix GFLAGS_INCLUDE_DIR in CMake

Summary: Currently on latest CentOS 8 stream dev server, gflags rpm have an invalid path. fb303 will produce an invalid path in the installed cmake. Anyone using this fb303 will then fail to compile.

Reviewed By: srikrishnagopu

Differential Revision: D41073188

fbshipit-source-id: 6ad4f60391c9253be614acc5d091236bf87d4697

Author: simuthus-fb

build/fbcode_builder/CMake/FindGflags.cmake

@@ -50,6 +50,7 @@ if (gflags_FOUND)
     # If gflags_INCLUDE_DIR does not actually exist, simply default it to
     # /usr/include on these systems.
     set(LIBGFLAGS_INCLUDE_DIR "/usr/include")
+    set(GFLAGS_INCLUDE_DIR "/usr/include")
   endif()
   set(LIBGFLAGS_FOUND ${gflags_FOUND})
   # cmake module compat