plugins: Turn on warnings

plugins: Turn on warnings

Turn on the same compiler warnings that were on for the library and fix
the issues.

  * signed/unsigned comparison
  * Missing function prototype
  * constify lms_plugin_info
  * Missing struct initializers (actually remove all of them)
  * Fix LMS_STATIC_STRING_SIZE to cast to char* since lms_string_size is
    declared with 'char *str' and we can't really change it (otherwise
    in other places it would not be able to change the
    string in place

Warnings like this in mp4 plugin is not fixed:
mp4/mp4.c:120:5: warning: implicit declaration of function
'MP4GetMetadataName' [-Wimplicit-function-declaration]
     MP4GetMetadataName(mp4_fh, &info.title.str);

This happens because MP4v2 lib deprecated the use of these functions in
favor to another MP4Tags*() API. Let this warning as is so we remember
to change this code.