Currently bpftrace uses only /lib/modules/`uname -r`/source/ if it
exists. This however leads to failures Debian where generated kernel
headers, such as include/generated/autoconf.h are put into a separate
directory, for example:

	---- 8< ---- (fuse_vs_mmapsem.bt)
	#include <linux/path.h>
	#include <linux/dcache.h>
	#include <linux/sched.h>
	#include <linux/mm_types.h>

	kprobe:fuse_readpage,kprobe:fuse_readpages {
	        printf("%s (%d) %s\n", probe, ((task_struct *)curtask)->mm->mmap_sem.count.counter, kstack);
	}
	---- 8< ----

	# ./bpftrace fuse_vs_mmapsem.bt
	/lib/modules/4.19.0-2-amd64/source/include/linux/kconfig.h:5:10: fatal error: 'generated/autoconf.h' file not found
	Unknown struct/union: 'task_struct'

Fix it by using both /lib/modules/`uname -r`/source/ and
/lib/modules/`uname -r`/build/ if both are present.

/cc @vincentbernat

P.S. I was not using C++ for ages, so please forgive me in advance if I
missed something.