printers: Fix compile warning

Missing GTK_LABEL cast.
11 jobs for <span class="ref-name">printers-mnemonic-warning</span> in 11 minutes and 43 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #208133
build

00:04:56

 
  Test
passed #208135
asan

00:06:47

passed #208134
test

00:04:11

 
  Manual
manual #208140
allowed to fail manual
armv8
manual #208136
allowed to fail manual
flatpak
manual #208139
allowed to fail manual
msan
manual #208141
allowed to fail manual
ppc64le
manual #208137
allowed to fail manual
tsan
manual #208138
allowed to fail manual
ubsan
 
  Review
manual #208142
allowed to fail manual
review
manual #208143
allowed to fail manual
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