libc: tag the Rune initialization function prototypes visibility as hidden.

It is good practice to export as few symbols as possible from your shared
libraries, so use the GCC visibility attribute in this case, matching what
Apple's libc does.

Reference:
https://developer.apple.com/library/mac/documentation/DeveloperTools/Conceptual/CppRuntimeEnv/Articles/SymbolVisibility.html

Hinted by: Apple's libc 1082.20.4
MFC after: 1 week