fix: int8_t is not considered an integer

fix: int8_t is not considered an integer

Add 'signed char' to the list of types we consider integers. Since 'char'
being signed or unsigned is implementation dependant, we have to
explicitly list 'char', 'signed char' and 'unsigned char' as integer
types.

Change-Id: I3aace2621f14f54e5d88d43de509a6cffde66c5b
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>