package/earlyoom: fix build with kernel < 4.3

PR_CAP_AMBIENT is only available since kernel 4.3 and
https://github.com/torvalds/linux/commit/58319057b7847667f0c9585b9de0e8932b0fdb08
resulting in the following build failure since bump to version 1.7 in
commit 07b7f886 and
https://github.com/rfjakob/earlyoom/commit/9edc5cfc9ad794ed59baec5e6ee8d1060456a315:

main.c: In function 'main':
main.c:119:11: error: 'PR_CAP_AMBIENT' undeclared (first use in this function)
     prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_CLEAR_ALL, 0, 0, 0);
           ^

Fixes:
 - http://autobuild.buildroot.org/results/5a0ad7e1f6de3c37e40c37d8f583fc699a457753



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>