arm64: compat: vdso: Expose BUILD_VDSO32

clock_gettime32 and clock_getres_time32 should be compiled only with the
32 bit vdso library.

Expose BUILD_VDSO32 when arm64 compat is compiled, to provide an
indication to the generic library to include these symbols.
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20190830135902.20861-2-vincenzo.frascino@arm.com

arch/arm64/include/asm/vdso/compat_gettimeofday.h

@@ -17,6 +17,7 @@
 #define VDSO_HAS_CLOCK_GETRES		1
 
 #define VDSO_HAS_32BIT_FALLBACK		1
+#define BUILD_VDSO32			1
 
 static __always_inline
 int gettimeofday_fallback(struct __kernel_old_timeval *_tv,