Fix removal of include and dev files
    
    The /usr/include directory will not necessarily exist within the
    buildroot, which will cause a build failure:
    
    + echo '=== REMOVE include and dev files ==='
    === REMOVE include and dev files ===
    + find /home/mcb30/rpmbuild/BUILDROOT/cross-binutils-2.39-1.fc38.x86_64/usr/include -type f -name '*.h' -delete
    find: '/home/mcb30/rpmbuild/BUILDROOT/cross-binutils-2.39-1.fc38.x86_64/usr/include': No such file or directory
    error: Bad exit status from /var/tmp/rpm-tmp.374lVg (%install)
    
    Fix by checking for the existence of the directory before attempt to
    find header files therein.
    
    Signed-off-by: Michael Brown <mbrown@fensystems.co.uk>