endiantest: Fix #if expression

Without this, `gcc -E endiantest.c` can fail and return the incorrect
endiannes as well as exiting with non-zero. Here is the actual error
shown in the output:

endiantest.c:2:31: error: #if with no expression
 #if __FLASHROM_LITTLE_ENDIAN__

I was able to reproduce this using gcc-6.3.0 and clang-4.0.1, but
newer compilers didn't have this issue.

Change-Id: Iba2febd861471ec821a494336e800c2564984332
Signed-off-by: David Hendricks <david.hendricks@gmail.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/43598
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
diff --git a/endiantest.c b/endiantest.c
index de44ec5..a73b908 100644
--- a/endiantest.c
+++ b/endiantest.c
@@ -1,5 +1,5 @@
 #include "platform.h"
-#if __FLASHROM_LITTLE_ENDIAN__
+#if defined(__FLASHROM_LITTLE_ENDIAN__)
 little
 #else
 big