gsm_29_118.h: Fix compilation with gcc-10

this causes problems when compiling user applications
/usr/bin/ld: ../../src/libvlr/libvlr.a(vlr_lu_fsm.o):/usr/local/include/osmocom/gsm/protocol/gsm_29_118.h:184: multiple definition of `sgsap_ie_tlvdef'; msc_main.o:/usr/local/include/osmocom/gsm/protocol/gsm_29_118.h:184: first defined here

Change-Id: Iaa1d36c7a9bb64aa84ee85fa3e40f6b3560fe693
diff --git a/include/osmocom/gsm/protocol/gsm_29_118.h b/include/osmocom/gsm/protocol/gsm_29_118.h
index 9adb90f..1583588 100644
--- a/include/osmocom/gsm/protocol/gsm_29_118.h
+++ b/include/osmocom/gsm/protocol/gsm_29_118.h
@@ -181,4 +181,4 @@
  * See also: RFC1123 Section 2.1 Host Names and Numbers */
 #define SGS_VLR_NAME_MAXLEN 255
 
-const struct tlv_definition sgsap_ie_tlvdef;
+extern const struct tlv_definition sgsap_ie_tlvdef;