Mark global variable as extern

Fixes compilation with GCC 10. (multiple definition of cmdline)

Author: dreua

src/intercept.h

@@ -215,7 +215,7 @@ bool is_overwritable_nop(const struct intercept_disasm_result *ins);
 
 void create_jump(unsigned char opcode, unsigned char *from, void *to);
 
-const char *cmdline;
+extern const char *cmdline;
 
 #define PAGE_SIZE ((size_t)0x1000)