buildsystem: Added file permissions to installed header files.

Headers were installed with executable flag set on Linux.

platform.mk

@@ -56,7 +56,7 @@ $(1)-install: $(2)/$(1)$(3)
 	$(INSTALL) $$^ $(PREFIX)/$(5)
 ifneq ($$(strip $$($(1)_HEADERS)),)
 	$(INSTALL) -d $(PREFIX)/$(INCLUDEDIR)/$(1)
-	$(INSTALL) $$($(1)_HEADERS) $(PREFIX)/$(INCLUDEDIR)/$(1)
+	$(INSTALL) -m 644 $$($(1)_HEADERS) $(PREFIX)/$(INCLUDEDIR)/$(1)
 endif
 .PHONY: $(1)-clean $(1)-install
 endef