Avoids warning about the annotation with GCC 10:

../src/gallium/frontends/osmesa/osmesa.c: In function ‘osmesa_choose_format’:
../src/util/compiler.h:84:21: warning: attribute ‘fallthrough’ not preceding a case label or default label
   84 | #define FALLTHROUGH __attribute__((fallthrough))
      |                     ^~~~~~~~~~~~~
../src/gallium/frontends/osmesa/osmesa.c:316:7: note: in expansion of macro ‘FALLTHROUGH’
  316 |       FALLTHROUGH;
      |       ^~~~~~~~~~~

Part-of: <mesa/mesa!10220>