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

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>