Bluetooth: Add convenience macros for common header sizes

The C pre-processor doesn't allow using sizeof() in comparisons such
as "#if FOO < sizeof(bar)". To make it possible to use such
comparisons where the sizes of headers are involved, introduce helper
macros for the headers instead of always having to hard-code magic
numbers into the code.

Change-Id: Iaf654cb4aaa49e83360901f5b01225ba4b952854
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>