Added -lm flag to link math library

[LordMhri]

Description

This PR adds the -lm flag to link math libraries in the C implementation of the Neural Network.

Error Details

Without the -lm flag , the following error occurs.

usr/bin/ld: build/neural.o: in function `sigmoid':
neural.c:(.text+0x3aa): undefined reference to `exp` 
clang: fatal error: linker command failed with exit code 1 (use -v to see invocation)

Changes Made

Adds -lm flag to the Makefile located in NeuralNetworkInAllLanges/C directory.

[dlidstrom]

Looks good, so now it compiles fine with Clang too. Thank you for your contribution!

[dlidstrom]

👍🏻