compat/pread.c: Add a forward declaration to fix a warning

read_in_full()'s is used in compat/pread.c.  read_in_full() is
declared in cache.h. But we can't include cache.h because too
many macros are defined there.  Using read_in_full() without
including cache.h is dangerous because we wouldn't recognize if
its prototyp changed.  gcc issues a warning about that.

This commit adds a forward declaration to git-compat-util.h.
git-compat-util.h is included by compat/pread.c _and_ cache.h.
Hence, changes in cache.h would be detected.

Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>

git-compat-util.h

@@ -185,6 +185,12 @@ extern int git_munmap(void *start, size_t length);
 #define pread git_pread
 extern ssize_t git_pread(int fd, void *buf, size_t count, off_t offset);
 #endif
+/*
+ * Forward decl that will remind us if its twin in cache.h changes.
+ * This function is used in compat/pread.c.  But we can't include
+ * cache.h there.
+ */
+extern ssize_t read_in_full(int fd, void *buf, size_t count);
 
 #ifdef NO_SETENV
 #define setenv gitsetenv