pw_checksum: CCITT CRC16 functions

Change-Id: I731445296690409cd97cad652f0827b50e36a58e
diff --git a/modules.gni b/modules.gni
index 8ab3503..d122bae 100644
--- a/modules.gni
+++ b/modules.gni
@@ -20,6 +20,7 @@
 dir_pw_bloat = "$dir_pigweed/pw_bloat"
 dir_pw_boot_armv7m = "$dir_pigweed/pw_boot_armv7m"
 dir_pw_build = "$dir_pigweed/pw_build"
+dir_pw_checksum = "$dir_pigweed/pw_checksum"
 dir_pw_cpu_exception = "$dir_pigweed/pw_cpu_exception"
 dir_pw_cpu_exception_armv7m = "$dir_pigweed/pw_cpu_exception_armv7m"
 dir_pw_docgen = "$dir_pigweed/pw_docgen"