crypto/chacha/asm/chacha-s390x.pl: add vx code path.

Signed-off-by: NPatrick Steuer <patrick.steuer@de.ibm.com>
Reviewed-by: NTim Hudson <tjh@openssl.org>
Reviewed-by: NRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6919)

crypto/chacha/build.info

@@ -9,6 +9,7 @@ GENERATE[chacha-armv4.S]=asm/chacha-armv4.pl $(PERLASM_SCHEME)
 INCLUDE[chacha-armv4.o]=..
 GENERATE[chacha-armv8.S]=asm/chacha-armv8.pl $(PERLASM_SCHEME)
 INCLUDE[chacha-armv8.o]=..
+INCLUDE[chacha-s390x.o]=..
 
 BEGINRAW[Makefile(unix)]
 ##### CHACHA assembler implementations