Remedy conflict between libzip and minizip zip.h.

minizip.pc.in would add @include@/minizip to the include path,
which would permit simply #include <zip.h> to use minizip. However
that conflicts with the zip.h from libzip that is put in the root
include directory. This now does not add /minizip to the include
path. Now when using pkg-config, #include <minizip/zip.h> must be
used, where #include <zip.h> would be used for libzip. This is an
incompatible change with the previous state. Users of minizip and
pkg-config will need to update their code. #include <unzip.h> will
need to be updated to #include <minizip/unzip.h> as well.

contrib/minizip/minizip.pc.in

@@ -1,7 +1,7 @@
 prefix=@prefix@
 exec_prefix=@exec_prefix@
 libdir=@libdir@
-includedir=@includedir@/minizip
+includedir=@includedir@
 
 Name: minizip
 Description: Minizip zip file manipulation library