Fix extern declarations based on Flexelint analysis

src/tus_hdr.h

@@ -26,29 +26,29 @@
  * SUCH DAMAGE.
  */
 
-const char * const hdr_resum;
-const char * const hdr_vers;
-const char * const hdr_ext;
-const char * const hdr_concat;
-const char * const hdr_chkalg;
-const char * const hdr_chksum;
-const char * const hdr_max;
-const char * const hdr_defer;
-const char * const hdr_meta;
-const char * const hdr_exp;
-const char * const hdr_cloc;
-const char * const hdr_loc;
-const char * const hdr_method;
-const char * const hdr_off;
-const char * const hdr_len;
+extern const char * const hdr_resum;
+extern const char * const hdr_vers;
+extern const char * const hdr_ext;
+extern const char * const hdr_concat;
+extern const char * const hdr_chkalg;
+extern const char * const hdr_chksum;
+extern const char * const hdr_max;
+extern const char * const hdr_defer;
+extern const char * const hdr_meta;
+extern const char * const hdr_exp;
+extern const char * const hdr_cloc;
+extern const char * const hdr_loc;
+extern const char * const hdr_method;
+extern const char * const hdr_off;
+extern const char * const hdr_len;
 
-const char * const hdr_origin;
-const char * const hdr_acah;
-const char * const hdr_acam;
-const char * const hdr_acma;
-const char * const hdr_acao;
-const char * const hdr_aceh;
+extern const char * const hdr_origin;
+extern const char * const hdr_acah;
+extern const char * const hdr_acam;
+extern const char * const hdr_acma;
+extern const char * const hdr_acao;
+extern const char * const hdr_aceh;
 
-const char * const hdr_allow;
+extern const char * const hdr_allow;
 
-const char * const hdr_vtc;
+extern const char * const hdr_vtc;