nimble/drivers/native: Add missing function stubs

This fixes unit test compilation error caused by missing
ble_phy_tifs_txtx_set() and ble_phy_encrypt_header_mask_set() function
stubs used by ble_ll_iso_big.c and ble_ll_isoal.c.
diff --git a/nimble/drivers/native/src/ble_phy.c b/nimble/drivers/native/src/ble_phy.c
index c38e6c8..5969dff 100644
--- a/nimble/drivers/native/src/ble_phy.c
+++ b/nimble/drivers/native/src/ble_phy.c
@@ -348,6 +348,11 @@
 }
 
 void
+ble_phy_encrypt_header_mask_set(uint8_t mask)
+{
+}
+
+void
 ble_phy_encrypt_iv_set(const uint8_t *iv)
 {
 }
@@ -651,3 +656,8 @@
 ble_phy_rfclk_disable(void)
 {
 }
+
+void
+ble_phy_tifs_txtx_set(uint16_t usecs, uint8_t anchor)
+{
+}