[BUG] pkgconfig benchmark.pc not modified with addition of -lpfm when BENCHMARK_ENABLE_LIBPFM=On

[Corin-EU]

When the option BENCHMARK_ENABLE_LIBPFM=On is used, the installed CMAKE files provide the necessary test to find libpfm when other software using cmake is being built with benchmark as a dependency.

However no requirement to include libpfm as a library is added to the pkgconfig benchmark.pc file is added.

So unless one manually appends -lpfm to the benchmark.pc line for Libs

--- benchmark.pc.ORIG      2025-01-17 21:05:44.622488319 +0100
+++ benchmark.pc        2025-01-17 21:07:35.562542402 +0100
@@ -7,6 +7,6 @@
Description: Google microbenchmark framework
Version: 1.9.1

-Libs: -L${libdir} -lbenchmark
+Libs: -L${libdir} -lbenchmark -lpfm
Libs.private: -lpthread 
Cflags: -I${includedir}

when building software which uses benchmark via pkg-config eg movit (high-performance, high-quality video filters) the following error for undefined references to pfm_get_os_event and pfm_initialize occurs when linking

        /usr/lib64/gcc/x86_64-suse-linux/13/../../../../x86_64-suse-linux/bin/ld: /var/tmp/root/BUILD/BUILD_movit_20250117_195233/ccAqTAKg.ltrans3.ltrans.o: in function `benchmark::internal::PerfCounters::Create(std::vector<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > > const&)':

       <artificial>:(.text+0x8445): undefined reference to `pfm_get_os_event_encoding' 
       /usr/lib64/gcc/x86_64-suse-linux/13/../../../../x86_64-suse-linux/bin/ld: <artificial>:(.text+0x894c): undefined reference to `pfm_initialize'

At first I tried appending -lpfm on the Libs.private line but that did not work but next test with it on the Libs line did work.