xmlrpc-c: install dev files only with 1st variant

xmlrpc-c: install dev files only with 1st variant

This adds a variant check to run InstallDev only when the first variant
is built.  Otherwise, a dependent package may install the default
version, even though the second variant ends up in the staging dir,
causing downstream packages to miss library dependencies, such as:

Package rtorrent-rpc is missing dependencies for the following libraries:
libxmlrpc_xmlparse.so.3
libxmlrpc_xmltok.so.3

Signed-off-by: Eneas U de Queiroz <redacted>