build: Fix build for systems with no /bin/true

There are systems with no /bin/true - it might have different path
(/usr/bin/true) or even a shell builtin.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/lib/Makefile.am b/lib/Makefile.am
index 0ec5b4c..d7c31ed 100644
--- a/lib/Makefile.am
+++ b/lib/Makefile.am
@@ -61,5 +61,5 @@
 else
 .PHONY: gitversion.h
 gitversion.h:
-	/bin/true
+	true
 endif