SecurityPkg: Update code to be more C11 compliant by using __func__

__FUNCTION__ is a pre-standard extension that gcc and Visual C++ among
others support, while __func__ was standardized in C99.

Since it's more standard, replace __FUNCTION__ with __func__ throughout
SecurityPkg.

Signed-off-by: Rebecca Cran <rebecca@bsdio.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>

SecurityPkg/VariableAuthenticated/SecureBootConfigDxe/SecureBootConfigImpl.c

@@ -167,7 +167,7 @@ GetCurrentTime (
     DEBUG ((
       DEBUG_ERROR,
       "%a(), GetTime() failed, status = '%r'\n",
-      __FUNCTION__,
+      __func__,
       Status
       ));
     return Status;