net/thunderx: add missing external dependency

net/thunderx: add missing external dependency

To prevent compile error when compiling for shared library:
nicvf_hw.o: In function `nicvf_qsize_regbit':
nicvf_hw.c:(.text+0xe64): undefined reference to `log2'
nicvf_hw.o: In function `nicvf_rss_reta_update':
nicvf_hw.c:(.text+0x19dd): undefined reference to `log2'
nicvf_hw.o: In function `nicvf_rss_reta_query':
nicvf_hw.c:(.text+0x1acb): undefined reference to `log2'

Fixes: e438796617dc ("net/thunderx: add PMD skeleton")

Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>