Fix missing header bug

[Cryptopractor]

The header file natpmp_declspec.h is included by natpmp.h but is not installed in the destination headers folder alongside natpmp.h. As a consequence, any code that includes natpmp.h will fail to compile, AC_CHECK_HEADER will also miss-report the header as not being present for the same reason.

This small fix will ensure the header is copied across when the library is installed.