Revert "build: link libfastjson against libm for modf()"

Pull Request rsyslog#167 modified libfastjson.pc to add an explicit dependency
on -lm through pkg-config. While this worked around missing modf() resolution
under glibc ≥ 2.42, it pushed the responsibility onto applications.

The correct solution is for libfastjson itself to link against libm so
that dependent applications do not need to adjust their linker flags.

This commit removes the pkg-config modifications introduced by commit 919f7d2.

Author: Whissi

libfastjson.pc.in

@@ -8,5 +8,5 @@ Description: a fast JSON implementation in C
 Version: @VERSION@
 Requires:
 Libs.private: @LIBS@
-Libs:  -L${libdir} -lfastjson -lm
+Libs:  -L${libdir} -lfastjson
 Cflags: -I${includedir}/libfastjson