libsigrokdecode.h: declare Windows dllexport for SRD_API routines

libsigrokdecode.h: declare Windows dllexport for SRD_API routines

Add the __declspec(dllexport) decoration to SRD_API routines on Windows
when building the library body (this is new). Stick with the default
visibility on all other platforms (as previously implemented).

This is similar to what Kyle Johnson submitted in PR 66, but phrased
differently. Avoid the non-portable indentation before the hash in
preprocessor instructions. Use positive logic for readability (put
the Windows branch next to the check for the Windows platform).

Reported-By: Kyle Johnson <redacted>