dma-mapping.h: Fix CRC mismatch issues from including genalloc.h

Commit 3a1bd738 ("dma-mapping-fast: Do not clobber existing atomic
pool") included the genalloc.h header file in the dma-mapping.h header
file.

Including genalloc.h in dma-mapping.h expands the definition of the
gen_pool structure for all files that include dma-mapping.h.
This can cause a different CRC value to be generated for an exported
function symbol, if the function parameters or any of their descendant
members contain genpool structures, as the genpool structures may have
been previously forward declared, but now they are fully defined.

Remove the inclusion of genalloc.h in dma-mapping.h to fix this.

Change-Id: I83a3fad284649b91372168d845f30d9302f75c1d
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>