libsync: ndk: Fix compilation in C mode.

Use of 'inline' without 'static' may allow the C compiler to uninline it
within the compilation unit, depending on the C standard level. Always
using 'static inline' avoids this problem.

Test: build + boot to launcher
Change-Id: Ifb6e1fa6b84286067ddc2daca4c8942c410e56ab