Add the x509_oid module to the build

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/library/Makefile b/library/Makefile
index 1c0e4d9..fb61911 100644
--- a/library/Makefile
+++ b/library/Makefile
@@ -198,6 +198,7 @@
 	   x509_crl.o \
 	   x509_crt.o \
 	   x509_csr.o \
+	   x509_oid.o \
 	   x509write.o \
 	   x509write_crt.o \
 	   x509write_csr.o \