po: Add P_() and C_() to dselect xgettext keywords

Make sure these will not get missed in case dselect happens to start
using them.

dselect/po/Makevars

@@ -8,7 +8,8 @@ subdir = dselect/po
 top_builddir = ../..
 
 # These options get passed to xgettext.
-XGETTEXT_OPTIONS = --from-code=UTF-8 --keyword --keyword=_ --keyword=N_
+XGETTEXT_OPTIONS = --from-code=UTF-8 --keyword --keyword=_ --keyword=N_ \
+                   --keyword=P_:1,2 --keyword=C_:1c,2
 
 # This is the copyright holder that gets inserted into the header of the
 # $(DOMAIN).pot file.  Set this to the copyright holder of the surrounding