Scope: Define GETTEXT_PACKAGE macro for recent cppcheck versions

Without this, cppcheck will throw errors like
store/scptreestore.c:1990:37: warning: syntax error [syntaxError]
  g_param_spec_boolean("sublevels", P_("Sublevels"),

This is probably caused by the P_() macro if GETTEXT_PACKAGE is not set.

scope/src/Makefile.am

@@ -60,6 +60,8 @@ scope_la_CFLAGS = $(AM_CFLAGS) $(VTE_CFLAGS) \
 	-Wno-shadow \
 	-I$(top_srcdir)/utils/src
 
+AM_CPPCHECKFLAGS = -DGETTEXT_PACKAGE="geany-plugins"
+
 include $(top_srcdir)/build/cppcheck.mk
 
 if UNITTESTS