Find libpfm dependency in installed CMake configs

Currently, Google Benchmark can be built and installed with support for
libpfm, but this can cause a problem if that installation is later
called upon by another CMake project. Indeed, while the installed CMake
configuration script correctly identifies that it needs to link against
libpfm, it doesn't try to find libpfm, meaning that the target will be
unavailable. This commit fixes this potential configuration-time error
by ensuring that an installation of Google Benchmark will correctly try
to find the libpfm dependency when it is used elsewhere.

cmake/Config.cmake.in

@@ -4,4 +4,8 @@ include (CMakeFindDependencyMacro)
 
 find_dependency (Threads)
 
+if (@BENCHMARK_ENABLE_LIBPFM@)
+    find_dependency (PFM)
+endif()
+
 include("${CMAKE_CURRENT_LIST_DIR}/@targets_export_name@.cmake")