make: Specify source files to include

Replace use of wildcard with list of source files to include. The `-y`
suffix is added, but has no significance since conditional compilation
has not been added yet.

Signed-off-by: Tim Crawford <tcrawford@system76.com>

src/common/common.mk

@@ -0,0 +1,5 @@
+# SPDX-License-Identifier: GPL-3.0-only
+
+common-y += i2c.c
+common-y += keymap.c
+common-y += version.c