make: Fixing missing math dependency in non-Windows json-c builds with meson

The -lm was missing in the pkgconfig file that was generated. This adds that similar to the CMake build system's output.

Signed-off-by: Tyler Erickson <tyler.erickson@seagate.com>
Signed-off-by: Sam James <sam@gentoo.org>

meson.build

@@ -265,6 +265,10 @@ endif
 # Include directories
 inc = include_directories('.')
 
+# Math library (needed for INFINITY, isnan, etc.)
+m_dep = cc.find_library('m', required: false)
+libjson_deps += [m_dep]
+
 # Build library
 libjson = library('json-c',
   sources,