tests/tcg: Fix the /proc/self/mem probing in the PROT_NONE gdbstub test

The `if not probe_proc_self_mem` check never passes, because
probe_proc_self_mem is a function object, which is a truthy value.
Add parentheses in order to perform a function call.

Fixes: dc84d50a7f9b ("tests/tcg: Add the PROT_NONE gdbstub test")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Message-Id: <20240131220245.235993-1-iii@linux.ibm.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

tests/tcg/multiarch/gdbstub/prot-none.py

@@ -20,7 +20,7 @@ def probe_proc_self_mem():
 
 def run_test():
     """Run through the tests one by one"""
-    if not probe_proc_self_mem:
+    if not probe_proc_self_mem():
         print("SKIP: /proc/self/mem is not usable")
         exit(0)
     gdb.Breakpoint("break_here")