Require framework directory to exist when building

The framework directory will be provided by a submodule.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/library/Makefile b/library/Makefile
index d11a98d..d5e4f64 100644
--- a/library/Makefile
+++ b/library/Makefile
@@ -1,3 +1,4 @@
+include ../framework/exported.make
 
 # Also see "include/mbedtls/mbedtls_config.h"