Degree symbol in Math

From LibreOffice 5.1 simply use the Unicode codepoint toggle in the edit
window of the Math formula editor.

To obtain any Unicode codepoint in the default font, Liberation Serif for
text, enter the code point and with cursor after the last character toggle
to display the glyph with <alt>+x. E.g. for U+00B0 DEGREE SIGN, enter
U+00B0 and then <alt>+x to toggle to °

You can also use the Symbols button and define a new entry in the Special
category for the degree sign, but I find the Unicode toggle to be more
direct.