ANDROID: sign-file: Fix warning when OPENSSL_NO_ENGINE is set

Place drain_openssl_errors() function under the same build constraints
as the code that calls it.

  scripts/sign-file.c:96:13: warning: unused function 'drain_openssl_errors' [-Wunused-function]
  static void drain_openssl_errors(void)
            ^
  1 warning generated.

For some reason this wasn't picked-up on during automated testing.

Fixes: e9d39639a5 ("FROMLIST: sign-file: Use OpenSSL provided define to compile out deprecated APIs")
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Change-Id: I3b337a9deac4ee83cb780792ece8f5f701a01f5f

scripts/sign-file.c

@@ -92,6 +92,7 @@ static void display_openssl_errors(int l)
 	}
 }
 
+#ifndef OPENSSL_NO_ENGINE
 static void drain_openssl_errors(void)
 {
 	const char *file;
@@ -101,6 +102,7 @@ static void drain_openssl_errors(void)
 		return;
 	while (ERR_get_error_line(&file, &line)) {}
 }
+#endif
 
 #define ERR(cond, fmt, ...)				\
 	do {						\