doxygen config: Update path to public headers

In d4043ee9, public headers
were moved from include/git to src/git, but the doxygen
configuration wasn't updated to reflect this. This patch
amends that slippage, making documentation generation once
again work as intended.

Signed-off-by: Andreas Ericsson <ae@op5.se>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

api.doxygen

 PROJECT_NAME = libgit2
 
-INPUT            = include
+INPUT            = src/git
 QUIET            = YES
 RECURSIVE        = YES
 FILE_PATTERNS    = *.h