Fix build on Windows

gcc couldn't find ghc_boot_platform.h. I'm not sure why it worked on
Linux.

compiler/ghc.mk

@@ -245,7 +245,7 @@ compiler/stage$1/build/Parser.y: compiler/parser/Parser.y.pp
 	$$(CPP) $$(RAWCPP_FLAGS) -P $$(compiler_CPP_OPTS) -x c $$< | grep -v '^#pragma GCC' > $$@
 
 compiler/stage$1/build/primops.txt: compiler/prelude/primops.txt.pp compiler/stage$1/$$(PLATFORM_H)
-	$$(CPP) $$(RAWCPP_FLAGS) -P $$(compiler_CPP_OPTS) -x c $$< | grep -v '^#pragma GCC' > $$@
+	$$(CPP) $$(RAWCPP_FLAGS) -P $$(compiler_CPP_OPTS) -Icompiler/stage$1 -x c $$< | grep -v '^#pragma GCC' > $$@
 
 ifneq "$$(BootingFromHc)" "YES"
 compiler/stage$1/build/primop-data-decl.hs-incl: compiler/stage$1/build/primops.txt $$(GENPRIMOP_INPLACE)