printf-posix: Fix mingw build
Commit
54615b95ff238e235e806855efc46a9abad09f2e changed the regular
expression for detecting C symbol prefixes but forgot to qoute square
brackets in the command line arguments for grep. That way when
building with mingw the condition was false although it ought to be
true instead. In particular scenarios this led to the following
compile error:
Cannot export rpl_printf: symbol not found
Cannot export rpl_scanf: symbol not found
collect2: error: ld returned 1 exit status
Fix this by properly quoting square brackets.