Disable parallel builds on win32 if MOZ_COVERAGE is set as it writes to .pdb files with each compile

config/rules.mk

@@ -1869,7 +1869,7 @@ FORCE:
 # so the files must be built serially
 # This requires a recent version of gmake 
 ifeq ($(OS_ARCH),WINNT)
-ifneq (,$(MOZ_DEBUG)$(MOZ_PROFILE))
+ifneq (,$(MOZ_DEBUG)$(MOZ_PROFILE)$(MOZ_COVERAGE))
 .NOTPARALLEL::
 endif
 endif