[bt][common] fix missing include in log.h

This header file provides the macro BT_DECLARE_FAKE_DRIVER, which
references the |zx_driver_rec_t| type. However, this header leaves
it for the client code to figure out which header declares that
type.

Improve the level of abstraction for clients of log.h, by including
<ddk/driver.h>, which declares the necessary type.

Bug: BT-726
Test: fx build (after an fx set without "--release")
Change-Id: Ifdeda67991c83ff1a5566cea7212274a48d21268
diff --git a/garnet/drivers/bluetooth/lib/common/log.h b/garnet/drivers/bluetooth/lib/common/log.h
index 742dda2..1714ffd 100644
--- a/garnet/drivers/bluetooth/lib/common/log.h
+++ b/garnet/drivers/bluetooth/lib/common/log.h
@@ -7,6 +7,8 @@
 
 #include <cstddef>
 
+#include <ddk/driver.h>
+
 // Logging utilities for the host library. This provides a common abstraction
 // over Zircon DDK debug utilities (used when btlib runs in a driver) and
 // printf.