package/paho-mqtt-c: fix build on musl

Set PAHO_HIGH_PERFORMANCE to disable free redefiniton as suggested by
upstream in https://github.com/eclipse/paho.mqtt.c/issues/846.

This will avoid the following build failure on musl:

/tmp/instance-1/output-1/host/x86_64-buildroot-linux-musl/sysroot/usr/include/sched.h:80:17: error: expected declaration specifiers or '...' before string constant
 void free(void *);
                 ^
/tmp/instance-1/output-1/host/x86_64-buildroot-linux-musl/sysroot/usr/include/sched.h:80:17: error: expected declaration specifiers or '...' before numeric constant
 void free(void *);
                 ^
[ 35%] Building C object src/CMakeFiles/common_obj.dir/Base64.c.o
[ 36%] Building C object src/CMakeFiles/common_obj.dir/SHA1.c.o
make[3]: *** [src/CMakeFiles/common_obj.dir/build.make:284: src/CMakeFiles/common_obj.dir/MQTTReasonCodes.c.o] Error 1

Fixes:
 - http://autobuild.buildroot.org/results//fbe57a1602fed331ddff3ff3560dce02573816ff



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>