Move comp/decomp alloc/free prototypes under guarding #ifndef

Function implemenations are already guarded by #ifndef MZ_NO_MALLOC, so
let's put prototypes under the same #ifndef. Also, while we are at it,
make those prototypes standard-compliant by adding void argument.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>

miniz_tdef.c

@@ -1555,6 +1555,7 @@ void *tdefl_write_image_to_png_file_in_memory(const void *pImage, int w, int h,
     return tdefl_write_image_to_png_file_in_memory_ex(pImage, w, h, num_chans, pLen_out, 6, MZ_FALSE);
 }
 
+#ifndef MINIZ_NO_MALLOC
 /* Allocate the tdefl_compressor and tinfl_decompressor structures in C so that */
 /* non-C language bindings to tdefL_ and tinfl_ API don't need to worry about */
 /* structure size and allocation mechanism. */
@@ -1567,6 +1568,7 @@ void tdefl_compressor_free(tdefl_compressor *pComp)
 {
     MZ_FREE(pComp);
 }
+#endif
 
 #ifdef _MSC_VER
 #pragma warning(pop)