[nrf noup] [nrfconnect] Remove D_SYS__PTHREADTYPES_H_ flag

... from the gnu17 configuration to avoid missing
pthread types definitions (_pthreadtypes.h).

This tricked the preprocessor the header was already
included, hence we didn't get its content.

Signed-off-by: Marcin Kajor <marcin.kajor@nordicsemi.no>

Author: markaj-nordic

config/nrfconnect/app/enable-gnu-std.cmake

@@ -1,6 +1,5 @@
 add_library(gnu17 INTERFACE)
 target_compile_options(gnu17
     INTERFACE 
-        $<$<COMPILE_LANGUAGE:CXX>:-std=gnu++17>
-        -D_SYS__PTHREADTYPES_H_)
+        $<$<COMPILE_LANGUAGE:CXX>:-std=gnu++17>)
 target_link_libraries(app PRIVATE gnu17)