curl-openssl.m4: do not add $prefix/include/openssl to CPPFLAGS

As OpenSSL's include files are all included using <openssl/*.h> in curl
source code, we just risk that existing openssl files will "shadow"
include files without path if that path is provided.

Fixes #9989
Closes #9988

m4/curl-openssl.m4

@@ -102,7 +102,6 @@ if test "x$OPT_OPENSSL" != xno; then
       SSL_LDFLAGS="-L$LIB_OPENSSL"
       SSL_CPPFLAGS="-I$PREFIX_OPENSSL/include"
     fi
-    SSL_CPPFLAGS="$SSL_CPPFLAGS -I$PREFIX_OPENSSL/include/openssl"
     ;;
   esac
 
@@ -150,7 +149,7 @@ if test "x$OPT_OPENSSL" != xno; then
      fi
      if test "$PKGCONFIG" = "no" -a -n "$PREFIX_OPENSSL" ; then
        # only set this if pkg-config wasn't used
-       CPPFLAGS="$CLEANCPPFLAGS -I$PREFIX_OPENSSL/include/openssl -I$PREFIX_OPENSSL/include"
+       CPPFLAGS="$CLEANCPPFLAGS -I$PREFIX_OPENSSL/include"
      fi
      # Linking previously failed, try extra paths from --with-openssl or
      # pkg-config.  Use a different function name to avoid reusing the earlier