cfg.mk: Add a syntax check for a redundant : in glnx_throw

This is going to be an easy bug to make when porting.

Example spotted by jlebon in 435c2dc35c (diff-dbc2359685171239c4708f2a8f6aa921)

Closes: #752
Approved by: jlebon

cfg.mk

@@ -23,6 +23,10 @@ local-checks-to-skip = \
     sc_prohibit_path_max_allocation \
     sc_trailing_blank \
 
+sc_glnx_errno_prefix_colon:
+	@prohibit='\<glnx_throw_errno_prefix *\(.*: ",' halt="don't add trailing : for glnx_throw_errno_prefix"	\
+	  $(_sc_search_regexp)
+
 #SHELL=bash -x
 show-vc-list-except:
 	@$(VC_LIST_EXCEPT)