Cleaning UEFI Build with additional OPENSSL_SYS_UEFI flags

Add OPENSSL_SYS_UEFI to remove unused syslog and uid stuffs for
more clean UEFI build.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2961)
diff --git a/crypto/uid.c b/crypto/uid.c
index 9756d51..4e1890f 100644
--- a/crypto/uid.c
+++ b/crypto/uid.c
@@ -19,7 +19,7 @@
     return issetugid();
 }
 
-#elif defined(OPENSSL_SYS_WIN32) || defined(OPENSSL_SYS_VXWORKS)
+#elif defined(OPENSSL_SYS_WIN32) || defined(OPENSSL_SYS_VXWORKS) || defined(OPENSSL_SYS_UEFI)
 
 int OPENSSL_issetugid(void)
 {