BUILD: makefile: make sure to also ignore SSL_INC when using wolfssl

I got a build error when adding USE_OPENSSL_WOLFSSL to my make command
line because SSL_INC was still set and caused some conflicting headers
to be included first. There's already an exclusion test for the wolfssl
variant used for SSL_LIB, make it also cover SSL_INC to avoid this.

This may be backported to 2.7 to ease testing of wolfssl.
1 file changed