Only typedef Ebl once.

Only typedef Ebl once.

Since commit 287a18452 libasm.h defines an opague Ebl handle.
This is fine, except for (internal) code that also includes libebl.h.
Since C11 having multiple typedefs for the same thing is fine, but we
do build using GNU/C99. This also allows multiple same typedefs, except
for (very) old GCCs.

This only affects internal code, since libebl.h isn't a public header.

For internal code, only add the typedef in libebl.h when libasm.h
hasn't been included. Make sure all code that includes both headers
includes libasm.h first.

Signed-off-by: Mark Wielaard <mark@klomp.org>