Revert "Guard visibility variables on AIX"

This reverts commit d785c29.

Author: jakeegan

MicroBenchmarks/libs/benchmark/CMakeLists.txt

@@ -51,10 +51,8 @@ option(BENCHMARK_USE_BUNDLED_GTEST "Use bundled GoogleTest. If disabled, the fin
 option(BENCHMARK_ENABLE_LIBPFM "Enable performance counters provided by libpfm" OFF)
 
 # Export only public symbols
-if(NOT ${CMAKE_SYSTEM_NAME} MATCHES "AIX")
-  set(CMAKE_CXX_VISIBILITY_PRESET hidden)
-  set(CMAKE_VISIBILITY_INLINES_HIDDEN ON)
-endif()
+set(CMAKE_CXX_VISIBILITY_PRESET hidden)
+set(CMAKE_VISIBILITY_INLINES_HIDDEN ON)
 
 if(MSVC)
     # As of CMake 3.18, CMAKE_SYSTEM_PROCESSOR is not set properly for MSVC and