fix typo in compiler

[SwagDevOps]

Using rubyc cloned from master:

Traceback (most recent call last):
        6: from /__enclose_io_memfs__/local/bin/rubyc:143:in `<main>'
        5: from /__enclose_io_memfs__/local/lib/compiler.rb:681:in `run!'
        4: from /__enclose_io_memfs__/local/lib/compiler.rb:808:in `prepare_local'
        3: from /__enclose_io_memfs__/local/lib/compiler/utils.rb:86:in `chdir'
        2: from /__enclose_io_memfs__/local/lib/compiler/utils.rb:86:in `chdir'
        1: from /__enclose_io_memfs__/local/lib/compiler/utils.rb:86:in `block in chdir'
/__enclose_io_memfs__/local/lib/compiler.rb:824:in `block in prepare_local': undefined local variable or method `gemspec' for #<Compiler:0x000055fb177aa928> (NameError)
Did you mean?  gemspecs

ruby-packer/lib/compiler.rb

Line 824 in e3d28dc

install_from_gemspec gemspec.first, gemfiles

Command used (nothing insane):

CPPFLAGS=-P /usr/local/bin/rubyc bin/sample -r . -d /tmp -o sample

Directory content:

drwxrwxr-x 2 dimitri dimitri 3,5K mai   12 12:11 bin
-rw-rw-r-- 1 dimitri dimitri 3,3K mai    8 11:22 gems.locked
-rw-rw-r-- 1 dimitri dimitri 1,4K mai    8 11:22 gems.rb
drwxrwxr-x 3 dimitri dimitri 3,5K mai   12 12:11 lib
-rw-rw-r-- 1 dimitri dimitri 1,9K mai   12 11:50 sample.gemspec

See #43