[C] Print ", ..." rather than ", ..." in diagnostics

[C] Print ", ..." rather than ",  ..." in diagnostics

pp_separate_with inserts a space after the separator, so there's
no need to add whitespace before "..." as well.

2019-09-30  Richard Sandiford  <richard.sandiford@arm.com>

gcc/c-family/
	* c-pretty-print.c (pp_c_parameter_type_list): Avoid printing
	two spaces between a comma and "...".

gcc/testsuite/
	* gcc.dg/Wincompatible-pointer-types-1.c (f1): Expect only one
	space between the comma and "...".

From-SVN: r276302