Fix build when valgrind header is present but does not define VALGRIND_MAKE_WRITABLE

[Vincent Pelletier]

Fixes gcc error:

signal_i386.o: In function `setup_exception':
/home/vincent/git/wine/dlls/ntdll/signal_i386.c:940: undefined
reference to `VALGRIND_MAKE_WRITABLE'
collect2: ld returned 1 exit status

Others uses of this constant are already checked this way.

diff --git a/dlls/ntdll/signal_i386.c b/dlls/ntdll/signal_i386.c
index 993936e..148c3a5 100644
--- a/dlls/ntdll/signal_i386.c
+++ b/dlls/ntdll/signal_i386.c
@@ -936,7 +936,7 @@ static EXCEPTION_RECORD *setup_exception( SIGCONTEXT *sigcontext, raise_func fun
     }
 
     stack--;  /* push the stack_layout structure */
-#ifdef HAVE_VALGRIND_MEMCHECK_H
+#ifdef VALGRIND_MAKE_WRITABLE
     VALGRIND_MAKE_WRITABLE(stack, sizeof(*stack));
 #endif
     stack->ret_addr     = (void *)0xdeadbabe;  /* raise_func must not return */