genksyms: rename m_abstract_declarator to abstract_declarator

This is called "abstract-declarator" in K&R. [1]

I am not sure what "m_" stands for, but the name is clear enough
without it.

No functional changes are intended.

[1] https://cs.wmich.edu/~gupta/teaching/cs4850/sumII06/The%20syntax%20of%20C%20in%20Backus-Naur%20form.htm



Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Nicolas Schier <n.schier@avm.de>