All installed headers and header directories of the project have
the `jerryscript-` prefix, except the math library, which installed
its `math.h` under `jerry-libm` directory.
This commit changes the name of the include directory to
`jerryscript-libm` to align it with the rest of the project.
JerryScript-DCO-1.0-Signed-off-by: Akos Kiss akiss@inf.u-szeged.hu