gcc 7 disable warning

GCC 7 generates (when compiling in C++11/14 mode) warnings such as:

    mangled name for ‘pybind11::class_<type_, options>&
    pybind11::class_<type_, options>::def(const char*, Func&&, const Extra&
    ...) [with Func = int (test_exc_sp::C::*)(int) noexcept; Extra = {};
    type_ = test_exc_sp::C; options = {}]’ will change in C++17 because the
    exception specification is part of a function type [-Wnoexcept-type]

There's nothing we can actually do in the code to avoid this, so just
disable the warning.

include/pybind11/pybind11.h

@@ -31,6 +31,7 @@
 #  pragma GCC diagnostic ignored "-Wmissing-field-initializers"
 #  pragma GCC diagnostic ignored "-Wstrict-aliasing"
 #  pragma GCC diagnostic ignored "-Wattributes"
+#  pragma GCC diagnostic ignored "-Wnoexcept-type"
 #endif
 
 #include "attr.h"