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 <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/19594
Signed-off-by: Robert Marko <[email protected]>