Fix compilation issue with new Log::Any

[basiliscos]

Hi!

With the patch I fix the following issue:

/tmp/1/MarpaX-Languages-C-AST/ prove -l --norc -r t/asm-opaque.t 
t/asm-opaque.t .. 1/2 
#   Failed test 'use MarpaX::Languages::C::AST;'
#   at t/asm-opaque.t line 8.
#     Tried to use 'MarpaX::Languages::C::AST'.
#     Error:  Log::Any::Proxy requires an 'category' parameter at /home/basiliscos/perl5/perlbrew/perls/perl-5.20.1/lib/site_perl/5.20.1/Log/Any.pm line 84.
# BEGIN failed--compilation aborted at /tmp/1/MarpaX-Languages-C-AST/lib/MarpaX/Languages/C/AST/Impl.pm line 36, <DATA> line 1.
# Compilation failed in require at /tmp/1/MarpaX-Languages-C-AST/lib/MarpaX/Languages/C/AST.pm line 12, <DATA> line 1.
# BEGIN failed--compilation aborted at /tmp/1/MarpaX-Languages-C-AST/lib/MarpaX/Languages/C/AST.pm line 12, <DATA> line 1.
# Compilation failed in require at t/asm-opaque.t line 8, <DATA> line 1.
# BEGIN failed--compilation aborted at t/asm-opaque.t line 8, <DATA> line 1.
Bailout called.  Further testing stopped:  
Can't locate object method "new" via package "MarpaX::Languages::C::AST" at
    t/asm-opaque.t line 13, <DATA> line 1 (#1)
    (F) You called a method correctly, and it correctly indicated a package
    functioning as a class, but that package doesn't define that particular
    method, nor does any of its base classes.  See perlobj.

Uncaught exception from user code:
    Can't locate object method "new" via package "MarpaX::Languages::C::AST" at t/asm-opaque.t line 13, <DATA> line 1.
# Looks like you planned 2 tests but ran 1.
# Looks like you failed 1 test of 1 run.
# Looks like your test exited with 255 just after 1.
FAILED--Further testing stopped.

WBR, basiliscos

[jddurand]

Many thanks - fix putted almost equivalent to yours - it appears that the only think I dislike in perl is its ability to automatically fill a hash, that's why I (ab)use exist() calls