gperftools: include headers in the staging_dir

gperftools: include headers in the staging_dir

This fixes version detection issues when other packages (like snort3)
try to find the tcmalloc library using CMake's find_package(). Without
the headers in the staging directory, CMake cannot read the version
information from tcmalloc.h, resulting in empty version strings.

Fixes:
Found TCMalloc: /builder/staging_dir/target-x86_64_musl/usr/lib/libtcmalloc.so (found version "")

Signed-off-by: Josef Schlehofer <pepe.schlehofer@gmail.com>