trusty-libc: Pull in libc-ext

We've added libc-ext for use in both kernel and userspace to add utility
functions (like scnprintf) that are not part of a standard libc but will
allow us to write safer/better code.

This patch pulls it into libc by default.

Bug: 133358839
Change-Id: Ied984f6a0025b0070ce6c87b953596cc698bb0c0
diff --git a/lib/libc-trusty/rules.mk b/lib/libc-trusty/rules.mk
index 87ad2b8..746f6bd 100644
--- a/lib/libc-trusty/rules.mk
+++ b/lib/libc-trusty/rules.mk
@@ -162,6 +162,9 @@
 MODULE_DEPS += trusty/kernel/lib/ubsan
 endif
 
+# Add Trusty libc extensions (separated due to use both in the kernel and here)
+MODULE_DEPS += trusty/kernel/lib/libc-ext
+
 # Add dependency on syscall-stubs
 include  trusty/user/base/lib/syscall-stubs/add-dependency-inc.mk