Fix "make test-cp-name-parser"

Fix "make test-cp-name-parser"

src/gdb/cp-name-parser.y: In function ‘int main(int, char**)’:
src/gdb/cp-name-parser.y:2132:30: error: ISO C++ forbids converting a string constant to ‘char*’ [-Werror=write-strings]
   char *str2, *extra_chars = "", c;
                              ^

Simply don't initialize the variable, it's not necessary.

gdb/ChangeLog:
2017-11-08  Pedro Alves  <palves@redhat.com>

	* cp-name-parser.y (main): Don't initialize extra_chars.