Fix backwards include of x509.h in oid.h

As agreed during the workshop, temporarily move definitions to oid.h even if
they might not semantically belong here, as a short-term measure allowing to
build libmbecrypto on its own (without X.509 files present in the source tree)
but still provide all the things Mbed TLS currently expects, and more
specifically preserve the API and ABI exposed by libmbedtls.
diff --git a/library/oid.c b/library/oid.c
index edea950..a7ee528 100644
--- a/library/oid.c
+++ b/library/oid.c
@@ -41,10 +41,6 @@
 #define mbedtls_snprintf snprintf
 #endif
 
-#if defined(MBEDTLS_X509_USE_C) || defined(MBEDTLS_X509_CREATE_C)
-#include "mbedtls/x509.h"
-#endif
-
 /*
  * Macro to automatically add the size of #define'd OIDs
  */
@@ -260,23 +256,23 @@
 {
     {
         { ADD_LEN( MBEDTLS_OID_BASIC_CONSTRAINTS ),    "id-ce-basicConstraints",   "Basic Constraints" },
-        MBEDTLS_X509_EXT_BASIC_CONSTRAINTS,
+        MBEDTLS_OID_X509_EXT_BASIC_CONSTRAINTS,
     },
     {
         { ADD_LEN( MBEDTLS_OID_KEY_USAGE ),            "id-ce-keyUsage",           "Key Usage" },
-        MBEDTLS_X509_EXT_KEY_USAGE,
+        MBEDTLS_OID_X509_EXT_KEY_USAGE,
     },
     {
         { ADD_LEN( MBEDTLS_OID_EXTENDED_KEY_USAGE ),   "id-ce-extKeyUsage",        "Extended Key Usage" },
-        MBEDTLS_X509_EXT_EXTENDED_KEY_USAGE,
+        MBEDTLS_OID_X509_EXT_EXTENDED_KEY_USAGE,
     },
     {
         { ADD_LEN( MBEDTLS_OID_SUBJECT_ALT_NAME ),     "id-ce-subjectAltName",     "Subject Alt Name" },
-        MBEDTLS_X509_EXT_SUBJECT_ALT_NAME,
+        MBEDTLS_OID_X509_EXT_SUBJECT_ALT_NAME,
     },
     {
         { ADD_LEN( MBEDTLS_OID_NS_CERT_TYPE ),         "id-netscape-certtype",     "Netscape Certificate Type" },
-        MBEDTLS_X509_EXT_NS_CERT_TYPE,
+        MBEDTLS_OID_X509_EXT_NS_CERT_TYPE,
     },
     {
         { NULL, 0, NULL, NULL },