build: Propagate function2

This is a useful header only dependency that is leveraged by many
projects. We will use it in future commits and we want dependees to be
able to use it.

Change-Id: Iea2856190678a2cfef742757ad96f9ea872377cc
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/.gitignore b/.gitignore
index be43dc8..61526e2 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,5 @@
 /build*/
 /subprojects/*
 !/subprojects/fmt.wrap
+!/subprojects/function2.wrap
 !/subprojects/googletest.wrap