Fix linking warnings on OS X with qmlimportscanner

The linker complains that some symbols were compiled with different visibility
settings when linking host_build tools such as the import scanner. As it turns
out, we do CONFIG += hide_symbols for static libraries (such as bootstrap or
qmldevtools) but naturally not for the final program source code. It appears
symbol visibility is not of importance for static libraries in host builds (as
opposed to static libraries later linked into shared libraries), therefore this
patch removes that.

Change-Id: I237a2d8669374eb059dc91b5378f6e3ec93d67a6
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>