build: Fix -Wconversion warnings for fpclassify et al

closes #8274

The parent commit tries a different approach, but that fails on Apple
Clang version:
    Apple LLVM version 10.0.0 (clang-1000.11.45.5)
    Target: x86_64-apple-darwin17.7.0
which somehow compiles the check_c_source_compiles() check, but then
complains during later compilation that __fpclassify is not defined
(regardless of "#include <math.h>").

src/nvim/strings.c

@@ -27,6 +27,7 @@
 #include "nvim/func_attr.h"
 #include "nvim/getchar.h"
 #include "nvim/mark.h"
+#include "nvim/math.h"
 #include "nvim/mbyte.h"
 #include "nvim/memfile.h"
 #include "nvim/memline.h"