glapi: Move declaration before code.

glapi: Move declaration before code.

This patch fixes the MSVC build error introduced by commit
673129e0b936b1c748e988d3f74f3efaab9e5693.

enums.c
mesa\main\enums.c(3776) : error C2143: syntax error : missing ';' before 'type'
mesa\main\enums.c(3781) : error C2065: 'elt' : undeclared identifier
mesa\main\enums.c(3781) : warning C4047: '!=' : 'int' differs in levels of indirection from 'void *'
mesa\main\enums.c(3782) : error C2065: 'elt' : undeclared identifier
mesa\main\enums.c(3782) : error C2223: left of '->offset' must point to struct/union
mesa\main\enums.c(3782) : warning C4033: '_mesa_lookup_enum_by_nr' must return a value

Signed-off-by: Vinson Lee <vlee@freedesktop.org>