perf: arm_cspmu: Fix modular builds due to missing MODULE_LICENSE()s

Building an arm64 allmodconfig target results in the following failure
from modpost:

  | ERROR: modpost: missing MODULE_LICENSE() in drivers/perf/arm_cspmu/arm_cspmu.o
  | ERROR: modpost: missing MODULE_LICENSE() in drivers/perf/arm_cspmu/nvidia_cspmu.o
  | make[1]: *** [scripts/Makefile.modpost:126: Module.symvers] Error 1
  | make: *** [Makefile:1944: modpost] Error 2

Add the missing MODULE_LICENSE() macros, following the license of the
source files and symbol exports.
Signed-off-by: Will Deacon <will@kernel.org>

drivers/perf/arm_cspmu/arm_cspmu.c

@@ -1299,3 +1299,5 @@ static void __exit arm_cspmu_exit(void)
 
 module_init(arm_cspmu_init);
 module_exit(arm_cspmu_exit);
+
+MODULE_LICENSE("GPL v2");

drivers/perf/arm_cspmu/nvidia_cspmu.c

@@ -396,3 +396,5 @@ int nv_cspmu_init_ops(struct arm_cspmu *cspmu)
 	return 0;
 }
 EXPORT_SYMBOL_GPL(nv_cspmu_init_ops);
+
+MODULE_LICENSE("GPL v2");