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>

package/tinydtls/tinydtls.mk

@@ -11,5 +11,7 @@ TINYDTLS_LICENSE = MIT
 TINYDTLS_LICENSE_FILES = tinydtls.h
 TINYDTLS_INSTALL_STAGING = YES
 TINYDTLS_STRIP_COMPONENTS = 2
+# use inttypes.h data types instead of u_intXX_t for musl compatibility
+TINYDTLS_CONF_ENV += CFLAGS="$(TARGET_CFLAGS) -DSHA2_USE_INTTYPES_H"
 
 $(eval $(autotools-package))