Remove redundant extern "C" from the definitions in printf.c

When using the library in C++, only the printf.h definitions need to be surrounded by `extern "C"` (see also this SO question). Apparently, some compilers (e.g. clang on Mac) [seem to have trouble](https://github.com/embeddedartistry/printf/commit/63d113c156557bfbe14079724ec88dbc6cbbd974#commitcomment-86463941) with definitions surrounded by it. So - let's remove it.