pps-tools: install timepps.h in location specified by RFC 2783

This is need by eg. gpsd to build with proper PPS support.
It was already submitted once in #2892 which ended in a stalemate.
Signed-off-by: Wolfgang Breyha <wbreyha@gmx.net>

utils/pps-tools/Makefile

@@ -33,8 +33,8 @@ endef
 TARGET_CFLAGS += -I$(STAGING_DIR)/usr/include
 
 define Build/InstallDev
-	$(INSTALL_DIR) $(1)/usr/include
-	$(CP) $(PKG_BUILD_DIR)/timepps.h $(1)/usr/include/
+	$(INSTALL_DIR) $(1)/usr/include/sys
+	$(CP) $(PKG_BUILD_DIR)/timepps.h $(1)/usr/include/sys/
 endef
 
 define Package/pps-tools/install