Update arglistfilter.py

Bug fix from over gllvm way found by @woodruffw

Author: ianamason

wllvm/arglistfilter.py

@@ -47,8 +47,8 @@ def __init__(self, inputList, exactMatches={}, patternMatches={}):
             '-v' : (0, ArgumentListFilter.compileOnlyCallback),
 
             #warnings (apart from the regex below)
-            '-w' : (0, ArgumentListFilter.compileOnlyCallback),
-            '-W' : (0, ArgumentListFilter.compileOnlyCallback),
+            '-w' : (0, ArgumentListFilter.compileUnaryCallback),
+            '-W' : (0, ArgumentListFilter.compileUnaryCallback),
 
 
             #iam: if this happens, then we need to stop and think.