nit: note expected behavior

nit: note expected behavior

If building for Windows with -Wimplicit-fallthrough you'll get a warning message about
"this statement may fall through."  Make it clear this is expected.

i686-w64-mingw32-gcc -c -Imbedtls/include -pipe -O2 -fstack-protector-strong -D_FORTIFY_SOURCE=2 -march=native -Wall -Wextra -Wno-missing-field-initializers -Wno-sign-compare -Wno-type-limits -Wno-unused-parameter -Wno-unused-but-set-variable -Wformat=2 -Wno-format-nonliteral -Wlogical-op -Wshadow -Wuninitialized -Winit-self -DNDEBUG -DWINVER=0x501  -I/source/pcre-8.44/ -I/source/mbedtls-2.16.10/include -I/source/brotli-1.0.9/c/include  -mwindows -Wall  w32log.c -o w32log.o
w32log.c: In function ‘LogWindowProc’:
w32log.c:1190:27: warning: this statement may fall through [-Wimplicit-fallthrough=]
 1190 |          g_bShowLogWindow = wParam;
      |          ~~~~~~~~~~~~~~~~~^~~~~~~~
w32log.c:1191:7: note: here
 1191 |       case WM_SIZE:  /* note: implicit-fallthrough */
      |       ^~~~