[gdb/testsuite] Fix unterminated string in i386-pkru.exp

[gdb/testsuite] Fix unterminated string in i386-pkru.exp

I ran into this error:
...
ERROR: tcl error sourcing gdb/testsuite/gdb.arch/i386-pkru.exp.
ERROR: missing "
    while executing
"untested ""
    invoked from within
"if { [prepare_for_testing "failed to prepare" ${testfile} ${srcfile} \
     [list debug additional_flags=${comp_flags}]] } {
    untested "failed to c..."
    (file "gdb/testsuite/gdb.arch/i386-pkru.exp" line 25)
    invoked from within
...
caused by:
...
    untested "failed to compile x86 PKEYS test.
...

Fix the unterminated string.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2019-07-26  Tom de Vries  <tdevries@suse.de>

	* gdb.arch/i386-pkru.exp: Fix unterminated string.