Include 'platform_def.h' header file in 'crash_reporting.S'

'crash_reporting.S' needs to include 'platform_def.h' to get the
definition of PLATFORM_CORE_COUNT.

Note: On FVP it was compiling because 'platform_def.h' gets included
through 'plat/fvp/include/plat_macros.S' but we don't want to rely on
that for other platforms.

Change-Id: I51e974776dd0f3bda10ad9849f5ef7b30c629833
Merge pull request #126 from sandrine-bailleux/sb/include-missing-hfile

Include 'platform_def.h' header file in 'crash_reporting.S'