[stdlib] fix prototype declaration in RuntimeShims.h

_swift_stdlib_getHardwareConcurrency's declaration isn't a proper prototype:
it's missing `void` inside the brackets.

Found while compiling the stdlib to WebAssembly, which fails with:

Functions with 'no-prototype' attribute must take varargs: _swift_stdlib_getHardwareConcurrency

This shouldn't impact existing platforms.

Author: zhuowei

stdlib/public/SwiftShims/RuntimeShims.h

@@ -57,7 +57,7 @@ SWIFT_RUNTIME_STDLIB_API
 int _swift_stdlib_putc_stderr(int C);
 
 SWIFT_RUNTIME_STDLIB_API
-__swift_size_t _swift_stdlib_getHardwareConcurrency();
+__swift_size_t _swift_stdlib_getHardwareConcurrency(void);
 
 /// Manually allocated memory is at least 16-byte aligned in Swift.
 ///