LibM: Declare rintl in math.h

Author: https://github.com/RealKC
Commit: 2d0f334e5d
Pull-request: https://github.com/SerenityOS/serenity/pull/5807
Reviewed-by: https://github.com/awesomekling

Userland/Libraries/LibM/math.h

@@ -234,6 +234,7 @@ long long llround(double) NOEXCEPT;
 long long llroundd(long double) NOEXCEPT;
 float rintf(float) NOEXCEPT;
 double rint(double) NOEXCEPT;
+long double rintl(long double) NOEXCEPT;
 long lrintl(long double) NOEXCEPT;
 long lrint(double) NOEXCEPT;
 long lrintf(float) NOEXCEPT;