package/wavemon: fix kernel header collision

Fix the following build failure raised since bump to version 0.9.4 in
commit 5cae1a0d:

In file included from iw_if.h:26:0,
                 from conf.c:19:
/home/buildroot/autobuild/instance-1/output-1/host/mips-buildroot-linux-gnu/sysroot/usr/include/linux/if.h:71:2: error: redeclaration of enumerator 'IFF_UP'
  IFF_UP    = 1<<0,  /* sysfs */
  ^
/home/buildroot/autobuild/instance-1/output-1/host/mips-buildroot-linux-gnu/sysroot/usr/include/net/if.h:44:5: note: previous definition of 'IFF_UP' was here
     IFF_UP = 0x1,  /* Interface is up.  */
     ^

Fixes:
 - http://autobuild.buildroot.org/results/cbdf3e0cf0bee8f1b076581768c24155afc320d9



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