Documentation: Adjust guidelines regarding math.h header

While libcamera prefers usage of the C standard library headers (xxx.h)
over the C++ version (cxxx), we make an exception for cmath as the
overloaded versions of the math functions are convenient. Document this,
and adjust checkstyle.py accordingly.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>

utils/checkstyle.py

@@ -244,9 +244,9 @@ class IncludeChecker(StyleChecker):
     patterns = ('*.cpp', '*.h')
 
     headers = ('assert', 'ctype', 'errno', 'fenv', 'float', 'inttypes',
-               'limits', 'locale', 'math', 'setjmp', 'signal', 'stdarg',
-               'stddef', 'stdint', 'stdio', 'stdlib', 'string', 'time', 'uchar',
-               'wchar', 'wctype')
+               'limits', 'locale', 'setjmp', 'signal', 'stdarg', 'stddef',
+               'stdint', 'stdio', 'stdlib', 'string', 'time', 'uchar', 'wchar',
+               'wctype')
     include_regex = re.compile('^#include <c([a-z]*)>')
 
     def __init__(self, content):