Declare lwlock.c's LWLockAcquireCommon() as a static inline.

68a2e52bbaf98f136 has introduced LWLockAcquireCommon() containing the
previous contents of LWLockAcquire() plus added functionality. The
latter then calls it, just like LWLockAcquireWithVar(). Because the
majority of callers don't need the added functionality, declare the
common code as inline. The compiler then can optimize away the unused
code. Doing so is also useful when looking at profiles, to
differentiate the users.

Backpatch to 9.4, the first branch to contain LWLockAcquireCommon().