Fix Makefile.am to remove non-existent Windows source file reference

Remove reference to src/async-process_windows.c from Makefile.am.
This file was removed in PR lem-project#26 when Windows support was ported to Lisp/CFFI.

Fixes build error: "make[1]: don't know how to make src/async-process_windows.c"

Author: Claude

Makefile.am

@@ -1,6 +1,6 @@
 pkglib_LTLIBRARIES = libasyncprocess.la
 libasyncprocess_la_LDFLAGS = -module -avoid-version -no-undefined
-libasyncprocess_la_SOURCES = src/async-process.c src/async-process_windows.c
+libasyncprocess_la_SOURCES = src/async-process.c
 
 copy:
 	$(MKDIR_P) static/`uname -m`/`uname`