Install inferno.a

The actual code archive is not installed by makefile which prevents dependencies to build native code.

I cannot create a PR so here is a patch.

Author: Joris Giovannangeli <joris.giovannangeli@ahrefs.com>
Date:   Wed Sep 18 16:07:57 2019 +0800

    install archive

diff --git a/src/Makefile b/src/Makefile
index 514711a..f883d9d 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -16,7 +16,7 @@ clean:
        rm -rf _build
 
 EXPORTED_SOURCES := $(wildcard lib/*.mli) $(wildcard lib/*Sig.ml)
-INSTALLED_FILES  := META $(wildcard _build/lib/inferno.cm*) $(EXPORTED_SOURCES)
+INSTALLED_FILES  := META $(wildcard _build/lib/inferno.cm*) _build/lib/inferno.a $(EXPORTED_SOURCES)
 
 install:
        @ if [ x$(PREFIX) = x ]; then                              ```