tinydtls: fix issue on u_intXX_t being undefined when building with musl

Fixes:
  http://autobuild.buildroot.net/results/928be69f90476e6b04be3a1afd3b74112bcac0a0



As mentioned in sha2/README, by default, tinydtls uses u_intXX_t data types
for 8 bit, 32 bit, and 64 bit unsigned integer type definitions.  To use
uintXX_t data types as defined by recent ANSI C standards and as included in
the inttypes.h header file, SHA2_USE_INTTYPES_H has to be define at compile
time.

[Peter: reword/simplify]
Signed-off-by: Fabrice Fontaine <fabrice.fontaine@orange.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>