headers: Consistent use of typeof and/or __typeof__.

headers: Consistent use of typeof and/or __typeof__.

Because of a copy-and-paste error, the Asterisk project was using __typeof
instead of typeof. It works because typeof, __typeof, and __typeof__ are
supported by GCC, but here the escaped variant was not intended. Therefore,
for consistence, we change this to typeof.

Change-Id: I2a962c3e596e882f691a19345445b14571a5f07c