graphics/mesa-devel: Fix 15-CURRENT 108de784513d build

108de784513d on main in src redefined CLOCK_BOOTTIME to alias
CLOCK_MONOTONIC, instead of CLOCK_UPTIME. This results in the following
errors when building this port under 15-CURRENT:

../src/intel/common/xe/intel_gem.c:72:9: error: duplicate case value '4'

72 |    case CLOCK_BOOTTIME:
   |         ^

/usr/include/sys/_clock_id.h:81:25: note: expanded from macro 'CLOCK_BOOTTIME'

81 | #define CLOCK_BOOTTIME          CLOCK_MONOTONIC
   |                                 ^

/usr/include/sys/_clock_id.h:56:26: note: expanded from macro 'CLOCK_MONOTONIC'

56 | #define CLOCK_MONOTONIC         4
   |                                 ^

../src/intel/common/xe/intel_gem.c:66:9: note: previous case defined here

66 |    case CLOCK_MONOTONIC:
   |         ^

/usr/include/sys/_clock_id.h:56:26: note: expanded from macro 'CLOCK_MONOTONIC'

56 | #define CLOCK_MONOTONIC         4
   |                                 ^

1 error generated.

On FreeBSD systems without 108de784513d the old definition will be used
while on 108de784513d and newer the duplicate case will be removed.

Reported by: pkg-fallout
Obtained from: src 676041c41ba5
Discussed with: imp