SHA256.cpp: Include some ZNC header first

This is required to make sure that zncconfig.h is the very first header that is
being included.

This problem was noticed by someone reporting some "macro redefined" warnings.

Signed-off-by: Uli Schlachter <psychon@znc.in>

SHA256.cpp

@@ -31,10 +31,10 @@
  * SUCH DAMAGE.
  */
 
-#include <string.h>
-
 #include "SHA256.h"
 
+#include <string.h>
+
 #define SHFR(x, n)    (x >> n)
 #define ROTR(x, n)   ((x >> n) | (x << ((sizeof(x) << 3) - n)))
 #define ROTL(x, n)   ((x << n) | (x >> ((sizeof(x) << 3) - n)))