Add two missing function definitions.
Add two missing function definitions.

This patch adds the function definitions for ivl_expr_branch and
ivl_expr_nature to the ivl.def file. This is needed to get Cygwin
and MinGW to compile correctly.

CastMi/iverilog,themperek/iverilog,themperek/iverilog,CastMi/iverilog,CastMi/iverilog,themperek/iverilog