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: e9d39639 ("FROMLIST: sign-file: Use OpenSSL provided define to compile out deprecated APIs")
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Change-Id: I3b337a9deac4ee83cb780792ece8f5f701a01f5f
(cherry picked from commit fad17703)