eal: fix C++ include for device event and DMA

eal: fix C++ include for device event and DMA

Currently the "extern C" section ends right before rte_dev_dma_unmap
and other DMA function declarations, causing some C++ compilers to
produce C++ mangled symbols to rte_dev_dma_unmap instead of C symbols.
This leads to build failures later when linking a final executable
against this object.

Fixes: a753e53d517b ("eal: add device event monitor framework")
Cc: stable@dpdk.org
Signed-off-by: Tianhao Chai <cth451@gmail.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Tyler Retzlaff <roretzla@linux.microsoft.com>