expprint: Fix format string warning

My compiler (gcc 5.4.0, clang 3.8) gives this warning:

/home/emaisin/src/binutils-gdb/gdb/expprint.c: In lambda function:
/home/emaisin/src/binutils-gdb/gdb/expprint.c:1055:35: error: format not a string literal and no format arguments [-Werror=format-security]
      fprintf_filtered (stream, mod);
                                   ^

Fix it by not using the passed string as the format string.

gdb/ChangeLog:

	* expprint.c (dump_subexp_body_standard): Use constant format
	string in fprintf_filtered call.