make prepareGlog idempotent
make prepareGlog idempotent

Summary:
The path to copy log_severity.h could refer to the
destination, which would result in an empty file being copied on some
rebuilds.

Reviewed By: willholen

Differential Revision: D15018283

fbshipit-source-id: 0081526a9686de8c74753738c165753de6dda18d

exponentjs/react-native,janicduplessis/react-native,facebook/react-native,exponent/react-native,javache/react-native,arthuralee/react-native,exponentjs/react-native,pandiaraj44/react-native,facebook/react-native,javache/react-native,hammerandchisel/react-native,facebook/react-native,hammerandchisel/react-native,janicduplessis/react-native,exponent/react-native,myntra/react-native,exponent/react-native,hoangpham95/react-native,myntra/react-native,arthuralee/react-native,facebook/react-native,facebook/react-native,pandiaraj44/react-native,pandiaraj44/react-native,javache/react-native,janicduplessis/react-native,pandiaraj44/react-native,janicduplessis/react-native,hoangpham95/react-native,myntra/react-native,hammerandchisel/react-native,janicduplessis/react-native,hoangpham95/react-native,exponent/react-native,hammerandchisel/react-native,hoangpham95/react-native,hammerandchisel/react-native,exponent/react-native,javache/react-native,myntra/react-native,exponentjs/react-native,janicduplessis/react-native,exponentjs/react-native,javache/react-native,exponentjs/react-native,hammerandchisel/react-native,pandiaraj44/react-native,hammerandchisel/react-native,arthuralee/react-native,hoangpham95/react-native,javache/react-native,hoangpham95/react-native,myntra/react-native,janicduplessis/react-native,exponentjs/react-native,facebook/react-native,myntra/react-native,myntra/react-native,javache/react-native,javache/react-native,myntra/react-native,hoangpham95/react-native,pandiaraj44/react-native,facebook/react-native,hoangpham95/react-native,exponentjs/react-native,exponentjs/react-native,facebook/react-native,myntra/react-native,arthuralee/react-native,javache/react-native,hammerandchisel/react-native,exponent/react-native,janicduplessis/react-native,exponent/react-native,facebook/react-native,pandiaraj44/react-native,arthuralee/react-native,pandiaraj44/react-native,exponent/react-native