rename context_size to ContextSize to match changes in FreeRDP

FreeRDP commit 208c9f844ad9d84333be474debc2a5282e9c4ad5 renames context_size to ContextSize in include/freerdp/freerdp.h, this breaks the build of the remmina rdp plugin at:

remmina-plugins/rdp/rdp_plugin.c:829:10: error: 'freerdp' has no member named 'context_size'

Downstream bug report with full build log and patch here:
https://bugs.gentoo.org/show_bug.cgi?id=476950