meson: Fix missing GSM_HEADER_IN_SUBDIR logic

Until now, this hadn't been translated from autoconf yet.
12 jobs for meson-fix-gsm-typos in 38 minutes and 47 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Preparation
passed #294371
gst indent

00:00:35

passed #294370
manifest

00:00:37

 
  Build
passed #294372
build fedora x86_64

00:05:28

passed #294373
build nodebug fedora x86_64

00:05:10

 
  Test
passed #294374
check fedora

00:03:24

passed #294375
integration testsuites fedora

00:08:27

 
  Full Builds
passed #294377
build cerbero android universal

00:11:06

passed #294378
build cerbero cross win32

00:07:35

passed #294379
build cerbero cross win64

00:05:45

passed #294376
build cerbero fedora x86_64

00:06:42

 
  Full Tests
passed #294380
valgrind bad

00:05:59

 
  Apps
passed #294381
android universal examples

00:07:08