Escape # in shell invocation

This fixes the following make error on Mac OS X versions that we don't
explicitly support yet due to missing sources:

    GNUmakefile:55: *** unterminated call to function `shell': missing `)'.
    Stop.

Reported by:	Justin Garrick

GNUmakefile

@@ -51,7 +51,7 @@ endif
 XNU_VERSION?=	$(shell uname -a|sed 's/^.*root:xnu-//g'|sed 's/~.*$$//')
 OSX_VERSION?=	$(shell sw_vers -productVersion)
 ifeq ($(wildcard xnu/xnu-$(XNU_VERSION)),)
-XNU_VERSION=	$(shell awk '/# $(OSX_VERSION)$$/ {print $$2}' xnu/GNUmakefile)
+XNU_VERSION=	$(shell awk '/\# $(OSX_VERSION)$$/ {print $$2}' xnu/GNUmakefile)
 endif
 ifneq ($(wildcard xnu/xnu-$(XNU_VERSION)),)
 FEATURES+=	-DHAVE_PF