gdb/testsuite: fix tcllint errors in gdb.base/startup-hints.exp

Fix:

    gdb/testsuite/gdb.base/startup-hints.exp:61:44: expression with substitutions should be enclosed by braces [unbraced-expr]
    gdb/testsuite/gdb.base/startup-hints.exp:64:28: expression with substitutions should be enclosed by braces [unbraced-expr]

Change-Id: Id21ff11ced82b11c11d0eb0d186fe6fc5c4c06c5