libwebsockets

Make LWS_SEND_BUFFER_PRE_PADDING preprocessor if-friendly

Commit 173e9c4e made LWS_SEND_BUFFER_SIZE a multiple of a certain
value returned by _LWS_PAD_SIZE macro. This macro expanded to
"sizeof(void *)" on non-x86_64 architectures, which made it
unsuitable to use LWS_SEND_BUFFER_SIZE in preprocessor #if
expressions in the library user code.

This patch preserves the padding logic since commit 173e9c4e but
makes it more preprocessor-friendly for applications using
libwebsockets by setting _LWS_PAD_SIZE to the size of "void *"
determined by cmake when libwebsockets is configured for the
target platform.

Signed-off-by: Alexander Lukichev <alexander.lukichev@gmail.com>