runtime(erlang): recognize -if/-elif as erlangPreCondit in syntax script

runtime(erlang): recognize -if/-elif as erlangPreCondit in syntax script

The -if(Condition)/-elif(Condition) are compiler macros that evaluate
the following lines only if Condition evaluates to true.  This patch
enables syntax highlighting for these macros.

https://www.erlang.org/doc/system/macros.html#conditional-compilation

closes: #18729

Signed-off-by: Vadim Yanitskiy <fixeria@osmocom.org>
Signed-off-by: Csaba Hoch <csaba.hoch@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>