Split env.hpp header from the ck.hpp header. (#2049)

* split env.hpp out of main headers

* fix namespace logic

[ROCm/composable_kernel commit: 572cd820ce]

include/ck/utility/env.hpp

@@ -184,4 +184,9 @@ void UpdateEnvVar(EnvVar, const std::string_view& val)
 }
 
 } // namespace ck
+
+// environment variable to enable logging:
+// export CK_LOGGING=ON or CK_LOGGING=1 or CK_LOGGING=ENABLED
+CK_DECLARE_ENV_VAR_BOOL(CK_LOGGING)
+
 #endif