fix segfault at startup

The previous changes to move the startup TCL code resulted in segfaults
during startup.  This seemingly innocuous patch fixes the problem.
I would explain why changing from 'foo[]' to '*foo' caused this issue,
but the difference seems superficial.  For now, this hot fix will do,
but this issue might bear further scrutiny.

src/openocd.h

@@ -37,6 +37,6 @@ void openocd_sleep_prelude(void);
 void openocd_sleep_postlude(void);
 
 /// provides a hard-coded command environment setup
-extern const char *openocd_startup_tcl;
+extern const char openocd_startup_tcl[];
 
 #endif