pru-software-support-package: pru_rpmsg_send missing 'const'

The function signature for pru_rpmsg_send (included in pru-software-support-package) is currently:

int16_t pru_rpmsg_send(struct pru_rpmsg_transport *transport, uint32_t src, uint32_t dst, void *data, uint16_t len);

However, this causes a compiler warning when passing in const data.

argument of type "const uint8_t *" is incompatible with parameter of type "void *"

This function signature can be updated to the following, which resolves the issue:

int16_t pru_rpmsg_send(struct pru_rpmsg_transport *transport, uint32_t src, uint32_t dst, const void *data, uint16_t len);