um: don't generate asm/bpf_perf_event.h

If we start validating the existence of the asm-generic side of
generated headers, this one causes a warning:

make[3]: *** No rule to make target 'arch/um/include/generated/asm/bpf_perf_event.h', needed by 'all'.  Stop.

The problem is that the asm-generic header only exists for the uapi
variant, but arch/um has no uapi headers and instead uses the x86
userspace API.

Add a custom file with an explicit redirect to avoid this.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

arch/um/include/asm/Kbuild

 # SPDX-License-Identifier: GPL-2.0
-generic-y += bpf_perf_event.h
 generic-y += bug.h
 generic-y += compat.h
 generic-y += current.h

arch/um/include/asm/bpf_perf_event.h

+/* SPDX-License-Identifier: GPL-2.0 */
+
+/*
+ * asm-generic/bpf_perf_event.h is part of the uapi headers, but since
+ * arch/um has no uapi of its on, we can't use the "generic-y"
+ * Kbuild rule to generate the wrapper
+ */
+
+#include <asm-generic/bpf_perf_event.h>