Fix zlibWrapper build

Fix zlibWrapper build

Just after a clone I'm getting this:

~/zstd/zlibWrapper$ cc -c zstd_zlibwrapper.o gz*.c -lz -lzstd -DSTDC
gzwrite.c: In function ‘gz_write’:
gzwrite.c:226:43: error: ‘z_uInt’ undeclared (first use in this
                         function); did you mean ‘uInt’?
  226 |             state.state->strm.avail_in = (z_uInt)n;
      |                                           ^~~~~~
      |                                           uInt
gzwrite.c:226:43: note: each undeclared identifier is reported only
                        once for each function it appears in
gzwrite.c:226:50: error: expected ‘;’ before ‘n’
  226 |             state.state->strm.avail_in = (z_uInt)n;
      |                                                  ^
      |                                                  ;

z_uInt is never used directly, zconf.h redefines uInt to z_uInt under
the condition that Z_PREFIX is set. All examples use uInt, and the type
of avail_in is also uInt.

In this commit I modify the cast to refer to the same type as the type
of lvalue.

Arguably, the real fix here is to handle possible overflows, but that's
beyond the scope of this commit.