chore: include debug symbols when doing debug builds

So I can see backtraces on Arch:

    yay -S gdb

Core dump files get written to `/var/lib/systemd/coredump/`, owned as
root, and zstd-compressed, so to read them, you have to do something
like:

    # unzstd /var/lib/systemd/coredump/core.nvim.1000.008a013d9f0d4e769d9dbcfdd9d9584a.13538.1656944479000000.std
    # chmod 666 /var/lib/systemd/coredump/core.nvim.1000.008a013d9f0d4e769d9dbcfdd9d9584a.13538.1656944479000000
    $ gdb nvim /var/lib/systemd/coredump/core.nvim.1000.008a013d9f0d4e769d9dbcfdd9d9584a.13538.1656944479000000
    (gdb) bt

Which shows (excerpted):

    Core was generated by `nvim'.
    Program terminated with signal SIGSEGV, Segmentation fault.

    warning: Section `.reg-xstate/13538' in core file too small.
    #0  match_thread (thread_args=0x55d4e0213230) at matcher.c:393
    393             haystack_t *match = args->corpus->haystacks[i];
    [Current thread is 1 (Thread 0x7fcda9b93740 (LWP 13538))]
    (gdb) bt
    #0  match_thread (thread_args=0x55d4e0213230) at matcher.c:393
    #1  0x00007fcda9e0f5ff in commandt_matcher_run (matcher=0x55d4e07eb9e0, needle=0x7fcda9e110f7 "stuff")
        at matcher.c:211
    #2  0x00007fcda9e0fb28 in commandt_temporary_demo_function () at matcher.c:312
    #3  0x00007fcda9bce309 in lj_vm_ffi_call () at buildvm_x86.dasc:2701
    #4  0x00007fcda9bf3b5e in lj_ccall_func (L=<optimized out>, cd=<optimized out>)
        at /build/luajit/src/luajit-2.0-4c2441c/src/lj_ccall.c:1167
    ...

lua/wincent/commandt/Makefile

@@ -7,7 +7,7 @@ CC = gcc
 CCFLAGS += -Wall -Wextra -Wno-unused-parameter
 
 ifdef DEBUG
-    CCFLAGS += -DDEBUG
+    CCFLAGS += -DDEBUG -g
 endif
 
 DLLEXT := so