Fix link failure with gcc 10

Without the 'extern' this looks like a definition not just a
declaration, in every file that includes the header. gcc 10 is stricter
about this kind of multiple definition.
12 jobs for gcc10 in 3 minutes and 39 seconds (queued for 28 seconds)
Status Job ID Name Coverage
  Docker Image
passed #1554793
debian-testing

00:00:21

 
  Build
passed #1554796
xserver-1.13

00:00:28

passed #1554797
xserver-1.14

00:00:22

passed #1554798
xserver-1.15

00:00:28

passed #1554799
xserver-1.16

00:00:57

passed #1554800
xserver-1.17

00:00:24

passed #1554801
xserver-1.18

00:00:29

passed #1554802
xserver-1.18-clang

00:00:27

passed #1554803
xserver-1.19

00:00:28

passed #1554804
xserver-1.19-clang

00:00:38

passed #1554794
xserver-1.20

00:01:08

passed #1554795
xserver-1.20-clang

00:00:44