Add missing $ to immediate in aes-gcm-avx10-x86_64.pl

Found by code review.  It worked anyway because the line is only
included in the Windows NASM file.  But omitting the $ is inconsistent.
No change to the generated asm files.

Change-Id: I12aa43fd9d71d7a71bbfdf2a47de7f23dd493cfa
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/74107
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
diff --git a/crypto/fipsmodule/modes/asm/aes-gcm-avx10-x86_64.pl b/crypto/fipsmodule/modes/asm/aes-gcm-avx10-x86_64.pl
index b65dee9..9ff7367 100644
--- a/crypto/fipsmodule/modes/asm/aes-gcm-avx10-x86_64.pl
+++ b/crypto/fipsmodule/modes/asm/aes-gcm-avx10-x86_64.pl
@@ -169,7 +169,7 @@
             my $pos     = 16 * $i;
             $code .= "movdqa $pos(%rsp), %xmm$reg_num\n";
         }
-        $code .= "add $alloc_size, %rsp\n";
+        $code .= "add \$$alloc_size, %rsp\n";
     }
 
     # Restore any general purpose registers that were saved earlier.