tools/expat: fix compilation with ccache

Even though expat is a C library, the configure script tests both C and
C++ compilers so a fix is needed for the latter.

Signed-off-by: Rosen Penev <rosenp@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/19594
Signed-off-by: Robert Marko <robimarko@gmail.com>
(cherry picked from commit b0650c50c8)
Link: https://github.com/openwrt/openwrt/pull/25329
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

tools/expat/Makefile

@@ -20,6 +20,7 @@ HOST_BUILD_PARALLEL:=1
 include $(INCLUDE_DIR)/host-build.mk
 
 HOSTCC := $(HOSTCC_NOCACHE)
+HOSTCXX := $(HOSTCXX_NOCACHE)
 
 HOST_CONFIGURE_ARGS += \
 	--disable-shared \