Replace cJSON with lightweight mkjson library

Based on PR #575 feedback, this commit replaces the large cJSON library
with the much smaller mkjson library for JSON output generation.

Changes:
- Replace cJSON (3,443 lines) with mkjson (357 lines) - ~10x reduction
- Optimize JSON output to only emit true boolean flags
- Use shorter field names (vid, pid, nports, ppps)
- Fix code style issues (remove stdbool.h, change ret to rc)
- Fix USB Full Speed detection for USB 2.0 devices
- Add USB speed reference citation
- Add missing newline at end of Makefile

The JSON output format remains compatible while significantly reducing
the library size as requested.

Author: benroeder

Makefile

@@ -37,7 +37,7 @@ else
 endif
 
 PROGRAM = uhubctl
-SOURCES = $(PROGRAM).c cJSON.c
+SOURCES = $(PROGRAM).c mkjson.c
 OBJECTS = $(SOURCES:.c=.o)
 
 all: $(PROGRAM)
@@ -55,4 +55,4 @@ install:
 clean:
 	$(RM) $(OBJECTS) $(PROGRAM).dSYM $(PROGRAM)
 
-.PHONY: all install clean
+.PHONY: all install clean