Add missing ".text" to Windows code for dummy_chacha20_poly1305_asm

This explicitly sets the section for the dummy_chacha20_poly1305_asm
symbol we generate for Windows. Without explicitly setting the
section, NASM would store it as an undefined symbol rather than a
defined symbol, resulting in a broken object file.

Change-Id: If8ff18f714be5e46af59ff869ae93e64e2c6e44b
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/37024
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

crypto/cipher_extra/asm/chacha20_poly1305_x86_64.pl

diff --git a/crypto/cipher_extra/asm/chacha20_poly1305_x86_64.pl b/crypto/cipher_extra/asm/chacha20_poly1305_x86_64.pl
index 5b2b977..ebf4563 100644
--- a/crypto/cipher_extra/asm/chacha20_poly1305_x86_64.pl
+++ b/crypto/cipher_extra/asm/chacha20_poly1305_x86_64.pl
@@ -2478,6 +2478,7 @@
   print $code;
 } else {
   print <<___;
+.text
 .globl dummy_chacha20_poly1305_asm
 .type dummy_chacha20_poly1305_asm,\@abi-omnipotent
 dummy_chacha20_poly1305_asm: