LoongArch: Fix build errors for tinyconfig

Building loongarch:tinyconfig fails with the following error.

./arch/loongarch/include/asm/page.h: In function 'pfn_valid':
./arch/loongarch/include/asm/page.h:42:32: error: 'PHYS_OFFSET' undeclared

Add the missing include file and fix succeeding vdso errors.

Fixes: 09cfefb7 ("LoongArch: Add memory management")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

arch/loongarch/include/asm/page.h

@@ -6,6 +6,7 @@
 #define _ASM_PAGE_H
 
 #include <linux/const.h>
+#include <asm/addrspace.h>
 
 /*
  * PAGE_SHIFT determines the page size

arch/loongarch/vdso/Makefile

@@ -21,6 +21,7 @@ ccflags-vdso += $(filter --target=%,$(KBUILD_CFLAGS))
 endif
 
 cflags-vdso := $(ccflags-vdso) \
+	-isystem $(shell $(CC) -print-file-name=include) \
 	$(filter -W%,$(filter-out -Wa$(comma)%,$(KBUILD_CFLAGS))) \
 	-O2 -g -fno-strict-aliasing -fno-common -fno-builtin -G0 \
 	-fno-stack-protector -fno-jump-tables -DDISABLE_BRANCH_PROFILING \