Add MacOSX10.15.sdk to Makefile.flags [ci skip]

This is necessary, but not sufficient (see #470), to be able to build OSVM on macOS Catalina.

build.macos64x64/common/Makefile.flags

@@ -7,7 +7,7 @@ XCODE:=$(shell /usr/bin/xcode-select -p)
 XCUB:=$(XCODE)/usr/bin
 SDKsDIR:=$(XCODE)/Platforms/MacOSX.platform/Developer/SDKs
 #Build oldest installed SDK (must be MacOSX10.12.sdk because of Metal and later)
-SDKs:=MacOSX10.12.sdk MacOSX10.13.sdk MacOSX10.14.sdk
+SDKs:=MacOSX10.12.sdk MacOSX10.13.sdk MacOSX10.14.sdk MacOSX10.15.sdk
 SDK:=$(firstword $(realpath $(addprefix $(SDKsDIR)/, $(SDKs))))
 ifeq ($(strip $(SDK)),)
 $(error could not find a valid SDK)