BCC_KERNEL_SOURCE doesn't work on OpenSUSE

On OpenSUSE 15.1 with kernel-devel and dependencies installed, running

`BCC_KERNEL_SOURCE=/lib/modules/4.12.14-lp151.28.48-default/build tools/opensnoop.py`

results in:

`:1:10: error: './include/linux/kconfig.h' file not found`

and the same with /lib/modules/.../source instead of .../build.

Running opensnoop.py without BCC_KERNEL_SOURCE in the env works.

Building a plain C module works via `make -C /lib/modules/.../build M=$PWD modules` . Adding an `#include ` in the module's source file also works.

This is with llvm-7 and clang-7 built from source as per INSTALL.md (the distro rpm's don't include libLLVMCore and others).

Reproduce the failure with tools/opensnoop.py and the BCC_KERNEL_SOURCE paths from the report, then read INSTALL.md and the kernel-source handling around that entry point. Compare the failing BCC invocation with the working plain C module and its linux/kconfig.h include; done means opensnoop.py runs successfully with BCC_KERNEL_SOURCE on OpenSUSE 15.1.

Written by the indexing model from the issue text.