contrib: png: Don't require runtime checks for NEON on arm on win32

NEON is always available there, just like on iOS.

This fixes building for Windows on ARM after switching to building
libpng with CMake - after that change, building failed due to
"No support for run-time ARM Neon checking; use compile-time options".

contrib/src/png/rules.mak

@@ -29,10 +29,13 @@ PNG_CONF += -DCMAKE_ASM_FLAGS="$(CFLAGS)"
 endif
 endif
 
-ifdef HAVE_IOS
 ifeq ($(ARCH),arm)
+ifdef HAVE_IOS
 # otherwise detection fails
 PNG_CONF += -DPNG_ARM_NEON=on
+else ifdef HAVE_WIN32
+# No runtime detection needed
+PNG_CONF += -DPNG_ARM_NEON=on
 endif
 endif