Network kit: Fix -Wformat-overflow

Increase array size, since gcc8 x86_64 warns 'sprintf' output
between 2 and 20 bytes into a destination of size 16
[-Werror=format-overflow=].

Change-Id: I641db97d963b64b0c3434cd498f29f4dcb61c373
Reviewed-on: https://review.haiku-os.org/472
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

src/kits/network/libnetapi/HttpRequest.cpp

@@ -1077,7 +1077,7 @@ BHttpRequest::_SendPostData()
 
 			if (fOptInputDataSize < 0) {
 				// Chunked transfer
-				char hexSize[16];
+				char hexSize[20];
 				size_t hexLength = sprintf(hexSize, "%ld", read);
 
 				fSocket->Write(hexSize, hexLength);