docs/scripts/gdb: add necessary make scripts_gdb step

In order to debug the kernel successfully with gdb you need to run
'make scripts_gdb' nowadays.

This was changed with the following commit:

Commit 67274c083438340ad16c ("scripts/gdb: delay generation of gdb
constants.py")

In order to have a complete guide for beginners this remark
should be added to the offial documentation.

Signed-off-by: Jakob Koschel <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/dev-tools/gdb-kernel-debugging.rst

···
   this mode. In this case, you should build the kernel with
   CONFIG_RANDOMIZE_BASE disabled if the architecture supports KASLR.
 
+- Build the gdb scripts (required on kernels v5.1 and above)::
+
+    make scripts_gdb
+
 - Enable the gdb stub of QEMU/KVM, either
 
     - at VM startup time by appending "-s" to the QEMU command line