Similarly to #1852 this is just a minor change with minimal impact :)

The -Wmissing-prototypes warning on Clang (or -Wmissing-declarations on GCC) is very useful to avoid accidents where a function definition in a source file doesn't match the corresponding declaration in a header file, as it would warn already during compilation and not fail much later during link time.

Unfortunately this means that exported functions defined only in the source file (usually the ones annotated with extern "C") will cause this warning to be emitted too (on Clang, GCC has a slightly different behavior with -Wmissing-declarations and doesn't warn here). For example:

magnum.cpp:211:1: warning: no previous prototype for function 'PyInit__magnum' [-Wmissing-prototypes]
PYBIND11_MODULE(_magnum, m) {
^
/usr/include/pybind11/detail/common.h:285:5: note: expanded from macro 'PYBIND11_MODULE'
    PYBIND11_PLUGIN_IMPL(name) {                                               \
    ^
/usr/include/pybind11/detail/common.h:172:42: note: expanded from macro 'PYBIND11_PLUGIN_IMPL'
    extern "C" PYBIND11_EXPORT PyObject *PyInit_##name()
                                         ^

This fixes the warning by providing a matching declaration right before the definition, both for the Python 3 and Python 2 variant.