sha1: add missing header guards

src/sha1.h

@@ -5,6 +5,9 @@
  * a Linking Exception. For full terms see the included COPYING file.
  */
 
+#ifndef INCLUDE_sha1_h__
+#define INCLUDE_sha1_h__
+
 typedef struct {
 	unsigned long long size;
 	unsigned int H[5];
@@ -19,3 +22,5 @@ void git__blk_SHA1_Final(unsigned char hashout[20], blk_SHA_CTX *ctx);
 #define SHA1_Init	git__blk_SHA1_Init
 #define SHA1_Update	git__blk_SHA1_Update
 #define SHA1_Final	git__blk_SHA1_Final
+
+#endif