fix gbuild precompiled check for --enable-symbol

its gb_SYMBOL not gb_SYMBOLS

Change-Id: Ia719d37d280498c856747d2806ec5a8a2c598a99
Reviewed-on: https://gerrit.libreoffice.org/55206Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Stahl <Michael.Stahl@cib.de>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

solenv/gbuild/PrecompiledHeaders.mk

@@ -23,7 +23,7 @@
 ifeq ($(gb_ENABLE_PCH),$(true))
 
 # gb_PrecompiledHeader_get_enableflags defined by platform
-ifneq ($(strip $(gb_DEBUGLEVEL)$(gb_SYMBOLS)),0)
+ifneq ($(strip $(gb_DEBUGLEVEL)$(gb_SYMBOL)),0)
 gb_PrecompiledHeader_DEBUGDIR := debug
 else
 gb_PrecompiledHeader_DEBUGDIR := nodebug