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.
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