[gdb/testsuite] Fix FAIL in gdb.base/printcmds.exp

[gdb/testsuite] Fix FAIL in gdb.base/printcmds.exp

With test-case gdb.base/printcmds.exp, I run into:
...
(gdb) print teststring2^M
$563 = (charptr) "more contents"^M
(gdb) FAIL: gdb.base/printcmds.exp: print teststring2
...

The corresponding test is:
...
     gdb_test "print teststring2" " = (charptr) \"more contents\""
...

Fix the FAIL by adding the missing quoting for the parentheses around charptr.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-03-14  Tom de Vries  <tdevries@suse.de>

	* gdb.base/printcmds.exp: Add missing quoting for "print
	teststring2".